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The Sun to the Earth – and Beyond: Panel Reports (2003)

Chapter: 2 Report of the Panel on Solar Wind and Magnetosphere Interactions

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Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
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Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 48
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 49
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 50
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 51
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 52
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 53
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 54
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 55
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 56
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 57
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 58
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 59
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 60
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 61
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 62
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 63
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 64
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 65
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 66
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 67
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 68
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 69
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 70
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 71
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 72
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 73
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 74
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 75
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 76
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 77
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 78
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 79
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 80
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 81
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 82
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 83
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 84
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 85
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 86
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 87
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 88
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 89
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 90
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 91
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 92
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 93
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 94
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 95
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 96
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 97
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 98
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 99
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 100
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 101
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 102
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 103
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 104
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 105
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 106
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 107
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
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Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 109
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 110
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
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Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
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Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
Page 113
Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
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Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
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Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
×
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Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
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Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
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Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
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Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
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Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
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Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
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Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
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Suggested Citation:"2 Report of the Panel on Solar Wind and Magnetosphere Interactions." National Research Council. 2003. The Sun to the Earth – and Beyond: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/10860.
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SUMMARY 49 2.1 SOLAR WIND-MAGNETOSPHERE INTERACTIONS: THE REALM OF MAGNETIZED PLASMAS 53 Introduction 53 The Fourth State of Matter 53 Reconnection 54 Flowing Magnetized Plasmas 55 The Storage and Release of Energy Coupling in a Collisionless Plasma Impact and Relevance 58 Summary 58 57 57 2.2 MAG N ETOSPH ERES AN D TH El R PARTS 58 Overview 58 Bow Shock 59 Magnetosheath 62 Magnetopause, Cusp, Boundary Layers 63 Magnetotai 1 66 I n ner Magnetosphere 70 Plasmasphere 72 Sol ar Wi nd I Interactions with Weakly Magnetized Bod ies 73 Outer Planets 83 2.3 PROCESSES 87 Introduction 87 The Creation and Annihilation of Magnetic Fields 87 Magnetospheres as Shields and Accelerators 88 Magnetospheres as Complex Coupled Systems 89 2.4 CURRENT PROGRAM 90 Introduction 90 Programs 91 Critical Needs 98 47

48 2.5 FUTURE PROJECTS 99 Introduction 99 Addressing the Major Themes 99 Project Summaries 100 Science Traceability 105 Prioritization: NASA and NSF 1 06 Prioritization of Other Agency and Interagency Initiatives 106 THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS TECHNOLOGY 1 08 Introduction 1 08 Propulsion Technology 1 09 S pacec raft Tech n o l ogy 1 1 1 Science Instrumentation Technology 1 1 3 Information Architecture Technology 1 1 4 Technology for Ground Systems and Operations 1 14 Recommendations and Priorities 1 1 4 2.7 SOLAR Wl N D-MAGN ETOSPHERE I INTERACTIONS: POLICY ISSU ES 1 1 5 I Introduction 1 1 5 Interagency Coordination 1 15 Coordination Between Programs and Divisions Within Agencies: NSF and NASA 117 Opportunities for Space Measurements in Entities Other Than NASA's Office of Space Science 118 Science in the Structure of Project Management 1 19 I International Cooperation 1 1 9 Model ing, Theory, and Data Assimi ration 1 20 Technology Development 1 21 Data Analysis, Dissemination, and Archiving 121 Extended Missions 1 22 ADDITIONAL READI NG 1 22

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS SUMMARY The study of solar wind-magnetosphere interactions at the turn of the 21 st century fi nds itself engaged in exciting exploration of exotic extraterrestrial environ- ments and consolidating a comprehensive, fundamental understanding of the terrestrial magnetosphere. To capi- talize on these discoveries, we need both classic mis- sions of exploration to the planets and modern multi- spacecraft probes in the near-Earth environment. This report summarizes what we now know about planetary magnetospheres and the processes within them, what we need to know, and how we should proceed in ob- tai n i ng th is knowledge. MAGNETIC FIELDS Magnetic fields play a crucial role in governing Earth's space environment. They organize the helio- spheric and magnetospheric plasmas, shield planetary bodies, such as Earth, from bombardment with charged particles, couple energy from one plasma regime to an- other, store that energy and later release it rapidly. More- over, they guide the motion of charged particles to re- gions where they can cause visible displays such as solar flares on the photosphere or the polar lights in the atmosphere. Partners in these processes are the plasmas, energetic particles, waves, and electromagnetic emis- sions from radio to x-ray wavelengths in the solar wind and the planetary magnetospheres. Solar and planetary magnetic fields organize space into normally well-separated regions. The principal plasma regimes are the corona, where the solar wind originates; the solar wind, the outward streaming plasma that carries the Sun's magnetic field to the outer helio- sphere; and the magnetospheres of planetary bodies, intrinsic or induced. The magnetospheres may act as flexible shields that deflect the solar wind and thereby protect the planet and its atmosphere from most of the direct impact of the solar wind particles. However, these shields are not impenetrable. One of the principal processes by which the shield is penetrated is called magnetic reconnection. This pro- cess is strongly controlled by the relative orientation of the magnetic fields in adjacent regions, leading to con- nection between the magnetosphere and the solar wind. Magnetic reconnection not only breaches the bound- aries between different plasma and magnetic field re- gions, it is also the main process involved in the rapid 49 release of magnetic energy in eruptions in the solar atmosphere and Earth's magnetosphere, in laboratory plasmas, and, presumably, in astrophysical settings. Other processes can breach the magnetic shield. In the case of weakly magnetized bodies such as comets, Venus and Mars, and the moons lo and Titan, neutral particle transport across plasma boundaries occurs, with subsequent ionization. In magnetically noisy environ- ments, particles can be scattered across the boundaries, and for small bodies finite gyroradius effects allow pen- etration. An important aspect of the plasmas in most of space is that the magnetic fields that guide the motion of the charged particles are, in turn, created by the motion of those very same particles. Thus the magnetized plasma can be quite nonlinear, enhancing, deflecting, or anni- hilating the original magnetic field. MAGNETOSPHERES Planetary magnetospheres are particularly acces- sible settings for studying the processes occurring in magnetized plasmas, providing unique insights into ba- sic physical processes not amenable to direct probing, processes such as particle acceleration, shock forma- tion, and magnetic reconnection. The solar wind inter- action with a magnetosphere produces thin boundaries, separating large regions of relatively uniform plasma. Within these thin boundaries microscale processes couple to the mesa- and macroscale processes, affecting the stability and dynamics not only of the thin boundary layer but also of the entire coupled magnetosphere system. The magnetospheric shields of planets and moons vary considerably. Some weakly magnetized planetary bodies like Earth's moon routinely lose their atmosphere to the solar wind, while others such asVenus and Mars have had thei r atmospheres sign if icantly altered, as indicated by the isotopic ratios of their atmo- spheric constituents, but not completely removed. Magnetospheres also exhibit rapid reconfigurations, such as the ejection of magnetic islands, or plasmoids, while the inner region collapses, as seen routinely in the tail regions of Earth and Jupiter. Overall planetary magnetospheres are complex, coupled systems, con- nected on one end to a supersonic flowing magnetized plasma, the solar wind, on the other end to a cold dense planetary atmosphere and ionosphere, and sometimes to embedded plasma sources such as satellites and rings. While each planetary magnetosphere presents great inter lectual chal lenges and its behavior provides insight into diverse astrophysical solar and laboratory systems,

50 the terrestrial magnetosphere is of particular practical interest. It provides a home to many technological sys- tems that are increasingly sensitive to magnetospheric disturbances. Such disturbances affect the quality of communications, our ability to navigate, the capacity of power transmission lines, the orbits of low-altitude sat- ellites, and the operation of geosynchronous spacecraft carrying TV broadcasts, relaying phone calls, and moni- toring our weather. Both astronauts and flight crews on polar air routes can receive undesirable levels of radia- tion from energetic particles controlled by the magneto- sphere. Thus, understanding and predicting the response of the magnetosphere to varying interplanetary condi- tions, i.e., space weather, has become a particular con- cern. THE TERRESTRIAL MAGNETOSPHERE The study of Earth's magnetosphere began with ground-based measurements of the time variations of the magnetospheric magnetic field. These observations revealed not only the existence of the magnetosphere but also its variable state of energization. The Interna- tional Geophysical Year initiated an era of discovery in which single-spacecraft missions throughout the mag- netosphere provided an overview of the characteristic regions, boundaries, and plasma conditions, with some evidence of the processes therein, but they did not elu- cidate how the processes in the magnetosphere work. Therefore, current and future exploration of the terres- trial magnetosphere concentrates on the use of multi- spacecraft missions complemented by ground-based arrays of magnetic, radar, and optical sensors to charac- terize plasma behavior in a dynamic environment and to probe cause and effect in a complex system at various scales. At the other planets, with few exceptions, we remain in the discovery phase since thus far we have generally been restricted to single-spacecraft missions, often si ngle flybys, not orbiters. There are many success stories in magnetospheric exploration as well as continuing puzzles. The standing bow shock is well understood, but it is only the fastest of three waves that should stand in the solar wind flow. The other two waves the intermediate mode, which rotates field and flow, and the slow mode, which "stretches" field lines could also lead to standing struc- ture. Reconnection is now known to provide a time- varyi ng i ntercon nection of the terrestrial magnetosphere with the magnetized solar wind, driving the circulation in the magnetosphere, but in a manner that is as yet not well understood. Reconnection is recognized to be the THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS principal mechanism for the violent release of stored magnetic energy and for magnetic flux return from the tails of the magnetospheres of both Jupiter and Earth. Nevertheless there is not agreement on what triggers the rapid onset of magnetotai I reconnection. Radial diffusion and pitch angle scattering of ener- getic particles apparently produce many of the observed features in the radiation belts of planetary magneto- spheres, but the driver of the radial diffusion remains elusive, and the sources and acceleration mechanisms for the involved energetic particles are not always clear. At unmagnetized planets the mechanism for the forma- tion of induced magnetospheres is relatively well under- stood but the atmospheric loss is poorly understood. INTRINSIC AND INDUCED Magnetospheres can be divided into two types: in- duced, if any intrinsic magnetic field of the body is so weak that the ionosphere is directly exposed to the flow- ing solar wind plasma, and intrinsic, if the body has an internal magnetic field sufficiently strong to deflect the plasma that flows against it. Induced magnetospheres form around highly electrically conducting obstacles if the conductor, generally an ionosphere, can stave off the solar wind flow. Induced magnetospheres also form in strong mass-loading environments such as at a rap- id Iy outgassi ng cometary nucleus. Comets, Venus, Mars, and some of the moons of the gas giants have magneto- spheres induced by the rotating magnetospheric plasma. Mercury, Earth, Ganymede, and the gas giants have in- trinsic magnetospheres. Circulation inside the intrinsic magnetospheres can be driven by the externally flowing plasma or by an internal source such as plasma derived from the volcanic gases of lo, accelerated by the rapidly rotating Jovian magnetosphere. The centrifugal force of this plasma drives a massive circulation pattern in the Jovian magnetosphere, powering a massive magneto- spheric "engine." Thus Jupiter acts as a bridge in our understanding of the terrestrial and astrophysical mag- netospheres. For both intrinsic and induced magnetospheres the supersonically flowing solar wind is deflected by the magnetosphere, forming a bow shock. Behind the bow shock, the decelerated shocked plasma flows around the obstacle in a region known as the magnetosheath. In intrinsic magnetospheres, the boundary between the flowing plasma of the solar wind and the plasma, con- nected by the magnetic field to the planet, is called the magnetopause. In an induced magnetosphere, the analo- gous boundary is called an ionopause. Often the mag-

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS netopause and the ionopause are thin layers of current. Behind the magnetosphere proper the magnetic field and plasma are stretched by the solar wind flow, form- ing a long magnetotail. Inside the magnetosphere, differ- ing regions of plasma can be found, such as the plasma- sphere in Earth's magnetosphere and the lo torus in Jupiter's. In an induced magnetosphere, the plasma is generally relatively cold and affected by the external flow in ways much different than in an intrinsic mag- netosphere. For these i educed magnetospheres, the so- lar wind interaction acts to scavenge the atmosphere and may be responsible for the loss of water from the atmospheres of Venus and Mars and for alteration of isotopic ratios over the eons since the formation of the solar system. THE PRESENT PROGRAM The present program of studies of magnetized space plasmas is robust. There is a vigorous program of ground-based measurements, theory, modeling, and data analysis, supported jointly by NASA, NSF, and, to a lesser extent, by other agencies. Data are being returned from the solar wind, magnetotail, magnetosphere, and low Earth orbit. Galileo has recently completed its ex- ploration of the Jovian magnetosphere and Cassini is on its way to Saturn. The data are being analyzed promptly, and significant scientific discoveries are being made. Several important projects are under development and moving toward their launch opportunities. Nevertheless, there is still much to do. UNIFYING THEMES The outstanding questions that need to be addressed in planetary magnetospheres can be divided into three themes: the creation and annihilation of magnetic fields; magnetospheres as shields and accelerators; and mag- netospheres as complex, coupled systems. The first theme includes the formation of the major magneto- spheric current systems: the magnetopause, the tail cur- rent, the ring current, and the field-aligned currents. This theme also includes the disruption of some of these cur- rents and reconnection of the magnetic field across cur- rent layers, at the magnetopause, in the magnetotail, and in planetary magnetodisks. Under the second theme is the role that induced and intrinsic magnetic fields play in deflecting the solar wind and the energetic particle populations coming from the Sun. These magnetospheres also store energy for 51 later release, leading to sudden energization of the plasma in the magnetosphere and acceleration of mag- netospheric energetic particles. In the inner magneto- sphere, trapped charged particles are also accelerated slowly to high energies by stochastic processes. None of these processes is well understood. Even less well un- derstood are the interactions of flowing magnetized plasma with the remanent fields of bodies like Mars. The third theme encompasses some of the most dif- ficult areas of magnetospheric research: the interactions among the d isparate pi asma regi mes with i n a magneto- sphere. The bow shock interacts with the incoming solar wind upstream and the magnetosheath and magneto- pause downstream. Reconnection changes the topology of magnetic field lines, connecting interplanetary and terrestrial magnetic field lines so that the plasmas from the two regimes mix, and allowing momentum and energy to flow into the magnetosphere from the solar wind. The ionosphere interacts with the polar magneto- sphere and the magnetospheric regions at lower latitudes. Planetary magnetospheres have their own unique twists on these processes. In the Jovian magneto- sphere the ionosphere enforces co-rotation of the plasma over enormous scales and a giant circulation pattern is set up within the magnetosphere. At the unmagnetized planets there is direct coupling of the solar wind with the neutral atmosphere. RECOMMENDATIONS The discipline of space physics and the subdisci- pline of solar wind-magnetosphere interactions have experienced an explosion of knowledge and understand- ing in recent years. Still there are some very basic pro- cesses that we do not understand, especially at a predic- tive level. If we cannot predict the rate of reconnection at our own magnetopause or in the magnetotail (and today we cannot), we have little hope of extending our knowledge to planetary and astrophysical systems. Thus we recommend that the future exploration of the ter- restrial and extraterrestrial magnetospheres should be directed toward the deeper understanding of the funda- mental physical processes and the global coupled sys- tems, supported and guided by theoretical investigations and simulation efforts. This requires multisatellite mis- sions and the optimal use of simultaneous, coordinated, and overlapping spacecraft missions. The global coupled system extends all the way down to the upper atmo- sphere and ionosphere. Thus in the terrestrial magneto- sphere ground-based facilities play an important part in the exploration of the coupled system.

52 In planning for the next decade of studies of solar wind-magnetosphere interactions we have been guided by four essentials. We must understand the physical pro- cesses involved and therefore need measurements with high resolution, capable of studying three-dimensional structure with support from theory and modeling. Our models must be predictive from knowledge of external cond itions. Th is req u i res gl obal, mu Iti poi nt observations and is best achieved with deep, theoretical insight rather than empirical models. We must investigate how re- gions couple, not simply how they work in isolation, and we must continue to explore new settings to de- velop greater understanding. Critical scientific objectives in the future exploration of solar system magnetospheres include the following: · A deeper physical understanding of fundamental plasma processes, such as particle acceleration, mag- netic reconnection, and the role of turbulence. Achieve- ment of this objective should be at the core of present and future space exploration, and the panel endorses the planned Magnetospheric Multiscale mission. · Understanding the scale sizes of the solar wind structures that power Earth's magnetosphere. Achieving this objective, which is needed for predictive purposes, wi I I requ i re mu Itispacecraft missions near 1 astronomi- cal unit (AU) with spacecraft separations measured in tenths of astronomical units. · Understanding the dynamics of the coupled mag- netospheric system and of space weather. Achievement of this objective requires arrays of instruments in space as well as on the ground (just as readings from ground weather stations are complemented by readings from space). A magnetospheric constellation of up to 100 spacecraft to monitor a significant volume of the magnetosphere is strongly recommended, along with complementary ground-based measurements. · Understanding the complex interaction between the solar wind and the polar ionosphere. Achievement of this objective requires the establishment at high lati- tudes of the long-awaited Advanced Modular Incoher- ent Scatter Radar (formerly known as the Relocatable Atmospheric Observatory). Th is faci I ity cou Id be en- hanced by many possible space missions, such as a stereo imager or a polesitter auroral imager. · Measurement of the density of the invisible popu- lations within the magnetosphere. To achieve this objec- tive, the panel recommends the establishment of mag- netometer arrays that can perform magnetoseismology, in analogy to terrestrial and solar seismology, recording transient waves and the ringing of the magnetosphere. THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS · Understanding the energization of the radiation belts. This long-sought objective requires knowledge of the radial swaths of the particle and field environment simultaneously at different local times and under differ- ent geomagnetic conditions to learn how and why par- ticle populations intensify and decay. · Understanding the complex interactions of the solar wind and planetary magnetospheres and atmo- spheres. To achieve this objective, particles and fields instruments will need to be flown on both Discovery- class and major space missions. · Understanding planetary magnetospheres. The exploration of planetary magnetospheres is in its infancy, yet comparisons between these magnetospheres and the terrestrial magnetosphere and with each other are criti- cal to fully understanding the processes taking place. Missions to study atmospheric loss fromVenus and Mars, the occurrence of lightning at Venus and Jupiter, the dynamics of Mercury's magnetosphere, and the joint control of the jovian aurora by lo and the solar wind are some of the many missions that could contribute to our understanding of planetary magnetospheres. TECHNOLOGY DEVELOPMENT While some of these objectives are already techni- cally within our grasp, additional technology develop- ment is needed for others. For example, several missions could be undertaken most effectively with a solar sail. Improved ion propulsion, nuclear-powered propulsion, and mid-size expendable launchers would also increase access to space. Smaller spacecraft systems and instru- ments would enable the constellation missions that are planned and would allow greater return from resource- limited planetary missions. Finally, attention needs to be given to the entire data chain, from operations to data transmission to their assimilation in models to reduce manpower and the total expense of the data chain. CHANGES IN POLICY Many of our programs would be enabled and en- hanced with some simple changes in policy. In some cases, this simply requires better coordination between or even within agencies. Sometimes data are obtained but funds are required for data access or archiving. We need to have processes to determine when a technique has moved from the research arena into the space- forecasting arena. We need to coordinate opportunities for access to space so that all such opportunities are

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS utilized, and we need to ensure that funding for space experiments is available when possible flight opportuni- ties arise. Presently, missions of opportunity are solicited far too seldom and on time scales incongruent with the duration of the opportunity. We also have to guard against using the space science budget to cover short- falls in other programs such as the space station. Budget raids can devastate smaller programs. Moreover, we need to find ways to reduce regulatory burdens, such as International Traffic in Arms Regulations (ITAR) and information technology security regulations, which have led to more and more obstacles to international collabo- ration and to university participation. These policies often have results much different than originally intended. High-level communication and coordination between regulatory agencies and NASA are needed to achieve reasonable implementation standards and procedures. SYNOPSIS In short, the research enterprise in solar wind-mag- netosphere interactions is strong, and much has been accomplished. Nevertheless, some very fundamental understanding is still needed to reach the quantitative level of a fully predictive science. Fortunately, the means to attain this understanding now exist. In some cases an investment in technology will bring us to the threshold of the needed breakthroughs. The next decade of this discipline, launched with the momentum of the last decade's discoveries, fueled by an exciting series of new observations, and supported by a strong program of theory and modeling, promises to usher in a new, quan- titative level of understanding of the Sun-Earth connec- tion. In the next section, the panel provides an overview of the workings of planetary magnetospheres. This over- view is followed in Section 2.2 by a detailed discussion of current understanding of the processes in the terres- trial magnetosphere and the environments of the plan- etary magnetospheres. This description is needed to understand why the panel has chosen the paths it rec- ommends, but it may be skipped by those seeking only to learn the recommendations. Section 2.3 is an attempt to provide three unifying themes that order the remain- ing tasks. Section 2.4 summarizes the existing program and presents recommendations. Sections 2.5, 2.6, and 2.7 describe, respectively, the recommended future pro- gram, the recommended technology developments, and the recommended policy changes that will enable the progress needed i n th is field. 53 2.1 SOLAR WIND- MAGNETOSPHERE INTERACTIONS: THE REALM OF MAGNETIZED PLASMAS INTRODUCTION Our i ncreasi ngly technological society rel ies more and more on assets launched into space. In addition, our investigations extend well past the local space plas- mas into those of the solar and astrophysical systems. Understanding the behavior of magnetized plasmas has become increasingly important. We must understand the environment in which our satellites operate. We need to predict how the solar wind affects the terrestrial mag- netosphere. We require insight into how the Sun gener- ates explosive events, and we desire to comprehend the workings of distant astrophysical systems that are clearly also affected by magnetic processes. The solar wind's interaction with the terrestrial and planetary magnetospheres allows us to treat a problem of much practical importance and learn how these plas- mas work in a most general manner. We can then ex- tend this knowledge to other plasma systems in regions we cannot probe directly. For users of this report who are not familiar with the physics of space plasmas, this section offers some brief insight into the basic plasma processes that occur in space. This section also provides a preview of the themes introduced in Section 2.3 and the rationale for the rec- ommendations made in subsequent sections. THE FOURTH STATE OF MATTER Plasmas are often referred to as the fourth state of matter. The behavior of this state, especially of magne- tized plasmas, can be nonintuitive. We are most familiar with the other three states solid, liquid, and gas- whose dynamical properties are governed by the differ- ing intermolecular forces of each state. Our intuition usually serves us well here. In an ideal gas, the forces between the molecu les are transmitted through col I i- sions. The random motions of the gas are characterized by a temperature and, in collisional equilibrium, all con- stituents come to the same temperature. The pressure in the gas is proportional to the product of the density and the temperature. A pressure gradient exerts a force. For example, in Earth's atmosphere we know that the pres- sure decreases with altitude. The force associated with

~ do. JO this pressure gradient acts on a parcel of air to support it agai nst the force of gravity. In space plasmas there often are no collisions in the usual sense. Thus, different components of the plasma can have d ifferent temperate res. Fu rther, temperate res along the magnetic field and across it can differ. Still, and counterintuitively, a plasma can exert pressure forces not only through the thermal motion of its par- ticles but also through its magnetic (and electric) fields. These fields do have pressure (proportional to the square of the field strength) and, as in the case of a gas, the gradient of that pressure exerts a force. In a magnetized plasma, the magnetic field orders the charged particle motion, the energy of the gyrating particles provides the plasma pressure perpendicular to the field, and the par- al lel thermal motions provide pressure along the field. An example of the interplay between these pres- sures is provided by the boundary between the solar wind flow and the magnetosphere. This region, the mag- netopause, is often treated as a boundary between a plasma with at most a weak magnetic field (the shocked solar wind) and a strong magnetic field (Earth's mag- netosphere) containing very little plasma. The pressure on the solar wind side is contained in the thermal mo- tions of the plasma. At the boundary of the plasma the thermal-pressure gradient exerts a force toward the mag- netosphere. The pressure in the magnetic field similarly exerts a force into the solar wind plasma, where the magnetic pressure decreases. Thus there is force bal- ance, and the magnetosphere and the solar wind estab- lish a pressure equilibrium in the absence of collisions. The ratio of the proton mass to the electron mass is 1,836. Thus an electron at the same temperature as a proton moves at 43 times the speed of the proton, and for this reason electrons can communicate rapidly in a plasma. Protons, though, have all the inertia and mo- mentum, and electrons tend to follow the dynamics of the protons, setting up small ambipolar electric fields to maintain quasi-neutrality in the plasma. As a result, ex- cept on the microscale, the electric field seldom builds up to such a degree that its pressure is important. How- ever, when electric fields do arise parallel to the mag- netic field they can be very important to the processes in the plasma, so much so that it is critical to be able to observe such generally small electric fields. The panel notes that these electric fields are frame independent, while the electric field in the direction perpendicular to the magnetic field is frame dependent, so that the per- pendicular electric field detected depends on the veloc- ity of the observer relative to the magnetic plasma. Thus the flowing solar wind has an electric field as seen in THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS Earth's reference frame. In short, while a plasma has many similarities to fluids and gases, it is different enough that our physical intuition is often ill-prepared to understand processes that occur therein. RECONNECTION The very large mass ratio between the proton and electron affects their gyromotion as well as their typical speeds. Because of the nature of the Lorentz force, which keeps a charged particle in orbit about a magnetic field, the radii of gyration of different charged particles are proportional to their mass times their velocity and in- versely proportional to their charge. If protons and elec- trons have the same energy perpendicular to the mag- netic field, the radius of the gyrating electron is 2.3 percent of that of the proton. In a collisionless plasma, the gyrating particles define the magnetic field lines. Particles in orbit about a magnetic field line stay with that magnetic field line. The ability of a charged particle to orbit a magnetic field line depends on the scale size for changes in the field. A small change can cause drift motion, and too large a change in the field on the scale of a gyroradius can cause a charged particle to become unmagnetized and move to orbit another field line. Owing to their smaller gyroradii, electrons can follow small-scale field variations to sizes roughly 43 times smaller than protons. One might think that this is moot for a system as large as Earth's magnetosphere, because its scale sizes are vast compared with those of the gyro- radii. In fact, it is standard practice to average over the gyromotion and treat the magnetized plasma as a mag- netic fluid. This formulation is known as magnetobydro- dynamics (MHD). However, the vast scale of the mag- netosphere does not allow us to completely ignore the kinetic motion of its particles. It just reduces the region in which that kinetic motion is crucial to small areas called neutral points. Close to these points the protons first become unmagnetized, and then closer yet the elec- trons become unmagnetized. This process in which charged particles lose their ability to define a magnetic field line is called reconnec- tion. If they are antiparallel, two neighboring magnetic field lines, say one that starts and ends on Earth and another that starts and ends on the Sun, can become connected so that two new field lines are created, both of which have one end on Earth and one end on the Sun. This topological change enables the plasmas in the two regions (terrestrial and solar in this case) to mix. It also allows momentum and energy to be supplied from one plasma to the other. Figure 2.1 illustrates the

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS geometry of this situation. The magnetic field at the re- connection point forms an x-type configuration with plasma flowing into it from the left (Earth's magneto- spheric plasma) and from the right (Sun's solar wind plasma). Field lines switch partners at the x-point, and plasma and cojoined field lines flow rapidly outward (top and bottom). Since an electric field is frame depen- dent, these moving magnetized plasmas have electric fields in the frame of the reconnection point, as sketched in Figure 2.1. Collisions, either particle-particle or wave-particle, can also demagnetize orbiting charged particles, and in numerical simulations numerical dissipation can mimic the reconnection process. Thus it is not always clear how the magnetosphere undergoes this most critical pro- cess. Much continued in situ study with high temporal Exhaust ZNIF .\ . Magnetosphere <~ \ \ YN ~ For i/ XNIF Exhaust Magnetosheath ME MINX N I F inflow FIGURE 2.1 The geometry of reconnecting antiparallel mag- netic fields at a neutral point. Courtesy of J.D. Scudder. 55 and spatial resolution is required, as well as investiga- tion with state-of-the-art numerical codes. From the above it is obvious that magnetic recon- nection is the crucial process enabling plasma (and momentum, energy, and magnetic flux) to cross magnetospheric boundaries. In addition, the partner- swapping process in reconnection can dramatically alter the stress balance in a plasma, leading to catastrophic energy release processes, such as solar flares and mag- netospheric substorms, discussed below. FLOWING MAGNETIZED PLASMAS A solid can support both compressional and trans- verse oscillations but a normal liquid and a gas cannot. Thus the dynamics of the flow around an object in a flowing gas is dominated by compressions and rarefac- tions. However, in a magnetized plasma that otherwise resembles a gas, there are transverse oscillations as well as two compressional waves. These three waves are usu- ally called fast, intermediate, and slow. They are all necessary to transmit an arbitrarily shaped perturbation through a magnetized plasma. For example, in the inter- action of the flowing solar wind with Earth's magneto- sphere, the fast mode slows, heats, and deflects the flow and magnetic field, but in general the intermediate mode is needed for additional field and flow deflection, and the slow mode is needed to prevent a density pileup at the subsolar point. Just as in a solid or gas, perturbations travel at a finite velocity, and in the magnetized plasma as in many other situations the velocity of each wave Separator mode is different. When it arrives at each of the planets the solar wind flow is supersonic, moving faster than the speed of the -YNIF compressional (fast mode) wave that could deflect it around the planetary obstacle. The momentum flux of the solar wind represents a dynamic pressure that con- fines the planetary magnetic field, but in order for it to be applied to the magnetosphere, the flow must pass through a bow shock that slows, deflects, and heats the flow, making it subsonic. Then the three wave modes (fast, intermediate, and slow) can act on the plasma to cause the deflection of the flow and alter the plasma conditions at the boundary of the magnetosphere. It is very important to magnetospheric processes that there are finite propagation times and finite transport times in the magnetosphere. When the solar wind mag- netic field reconnects with the subsolar magnetospheric magnetic field, it begins to transport magnetic flux to the geomagnetic tail, as illustrated in Figure 2.2. The tail may increase in size for about an hour. Then, when

56 Inter~lanetarv Polar Cusp -Magnetopause Current FIGURE 2.2 A cutaway diagram of Earth's magnetosphere.Cour- tesy of C.T. Russell, University of California, Los Angeles. reconnection begins in the tail, it may require another hour to transport the magnetic flux out of the tail, but the signals denoting the onset of each reconnection event can travel through the magnetosphere within minutes. Other manifestations of the finite travel time of perturba- tions in the plasma include the resonant behavior of individual dipole flux tubes, with waves bouncing back and forth in the magnetic tube and the propagation of shock-initiated disturbances through the magnetosphere. These signals can be used to probe the magnetosphere much as seismology uses waves triggered by earth- quakes to probe the structure of Earth. The waves discussed above have very long wave- lengths, generally a large fraction of the dimension of the system. Waves at shorter wavelengths are also pres- ent in magnetized plasmas. Often these waves are responsible for releasing free energy from the plasma, ultimately into heating of the system. Examples of such waves include those caused by the upstream ions reflected at the bow shock and ion cyclotron waves produced both in the solar wind interaction with comets and in the flow of the lo torus past the mass-loading region at lo. THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS When a flowing magnetized plasma interacts with an unmagnetized planet there are important simi rarities and differences from the magnetized case. First, if the unmagnetized planet has an atmosphere, then it will be ionized by the solar UV and EUV radiation. The solar wind magnetic field will be draped across this electri- cally conducting ionosphere and will pile up in front of it, as illustrated in Figure 2.3. This pileup region deflects the solar wind particles around the ionospheric obstacle while the magnetic field begins to diffuse into the iono- sphere. However, the solar wind magnetic field is quite variable in direction, and the long-term (days) vector average field is close to zero. The diffusion time into the interior of the ionosphere, high in the collisionless exo- sphere, is long. Thus, the deep ionosphere does not generally become strongly magnetized by this external magnetic field. Second, the neutral atmosphere often has a great enough extent that the neutral density is significant on solar wind stream tubes that are flowing rapidly. When the neutral atoms and molecules of the atmosphere become ions they are accelerated by the solar wind and lost to the planet. This loss can lead to significant changes in a planetary atmosphere over the age of the solar system. Hence the magnetic field is both an accelerator and a shield at unmagnetized planets. tow Shank Shock s (Outside Tail) FIGURE 2.3 The solar wind interaction with an unmagnetized planet, illustrating the effect of the shock on the field and the stretching of the field to form an induced magnetotail. Courtesy of C.T. Russell, University of California, Los Angeles.

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS THE STORAGE AND RELEASE OF ENERGY The interaction of the solar wind with a planet cre- ates a magnetic cavity with a long magnetic tail. Since a magnetic field has an energy density, the formation of a planetary magnetotail or its expansion involves the stor- age of energy. The additional energy in a magnetotail can be provided externally when the magnetosphere reconnects with the interplanetary magnetic field at the dayside if the fields are in opposite directions. The stored energy is extracted from the mechanical energy of the solar wind flow as the magnetic field lines joining the magnetosphere to the solar wind slow its flow. The two nearly antiparallel magnetic lobes of the tail can also reconnect in a manner similar to the fields at the magnetopause, and energy can be released rap- idly from the tail lobes, in a process called a substorm. Some of this energy accelerates the bulk of the plasma, some heats the plasma, and some energizes a few par- ticles to very high energies. On Earth these high-energy particles help populate the radiation belt. Over the au- roral zones beams of particles are created that cause the auroral emissions during the reconfiguration of the night magnetosphere associated with these acceleration pro- cesses. On the Sun, solar flares are seen when highly energetic particles strike the solar surface after a re- connection event in the magnetic field above the photo- sphere. The creation of a small number of energetic par- ticles (a "high-energy tail," as these energetic particles are often called) is another nonintuitive phenomenon in a collisionless plasma. In a collisional environment the number of particles as a function of energy follows a Maxwellian distribution in which there are very few par- ticles that deviate much from the bulk of the distribu- tion. However, in a collisionless plasma, the high-en- ergy particles can receive a disproportionate amount of the energy being put into the plasma. Understanding the conditions under which this occurs is important, be- cause these high-energy particles can be deleterious to a spacecraft system in orbit in the magnetosphere. In the broader astrophysical setting, we are interested in un- derstanding how cosmic rays reach energies much, much higher than those of particles accelerated by pro- cesses in the solar system. COUPLING IN A COLLISIONLESS PLASMA The large proton-to-electron mass ratio not only affects the relative speeds of the two particles under normal circumstances and also their relative gyroradii, 17 .~ . but also the charge separation. In general, electrons will stay close to the ions to maintain charge neutrality. Al- though in the absence of collisions charged particles will stay on a single magnetic field line, any charge imbalances that arise can generally be removed by mo- tion along the magnetic field. Thus most plasmas are quasi-charge-neutral. The magnetic field in most space plasmas is strong enough to divide space into different plasma regimes with little communication across the boundaries be- tween them. When the magnetic fields in two adjacent regions are in nearly opposite directions, reconnection, discussed above, can occur, linking the two regions. If one of these regions is flowing past the other, this link- age can transfer momentum from the flowing plasma to the initially stationary plasma. This is the way in which Earth's magnetosphere is stirred by the solar wind that flows past it. Surface waves can also transfer momentum across such a boundary if the system is dissipative. The waves may be generated at the bow shock and blown back against the magnetopause, or they may arise in the interaction due to a velocity-shear instability, such as the Kelvin-Helmholtz instability, that is akin to the pro- cess by which the wind creates ocean waves. An important coupling occurs between the plasma and the neutral gas in Earth's magnetosphere at the foot of the field line. The stress that is applied at the interface between the solar wind and the magnetosphere must eventually be taken up by Earth, and this occurs ulti- mately through collisional transfer between the ions and the neutrals. To get the ions moving at the feet of mag- netic field lines and overcome the drag of the neutral atmosphere, the magnetosphere sets up a large current system that connects the outer magnetosphere with the ionosphere along magnetic field lines. The closure of the current across field lines in the collisional, electri- cal Iy conducting ionosphere accelerates the low-altitude ions via the J x B force or ponderomotive force. This force is the macroscale manifestation of the Lorentz force, which maintains charged particles in their orbits around magnetic field lines. It can also transfer stress from the ionosphere to the magnetosphere such as to enforce co-rotation of the cold magnetospheric ions. This mechanism is especially important in the jovian magnetosphere. In the terrestrial auroral ionosphere, the chain of momentum transfer is completed when the moving ionospheric ions transfer their momentum to the neutral gas, generating high-altitude winds. The overall coupling from the solar wind down to the ionosphere is a very complex process, and it is fair to say that while we now understand this process much better than even

58 a decade ago, it is not yet understood as well as we need to understand it. IMPACT AND RELEVANCE Planetary magnetospheres are important first and foremost because we live inside one. Earth is currently the only habitable planet, and that habitability may in part be due to the shield provided by Earth's magneto- sphere. That shield also protects our spacecraft and com- munication systems. At times, however, the shield is overwhelmed and it is not as effective. It is becoming more and more important to be able to predict when these ti mes of stress occur. Now the question of habitability extends beyond Earth. Other planets are magnetized and may once have had Earth-like fields even if they do not today. Could an early magnetic field on Mars, say, have protected the atmosphere and climate of Mars from the solar wind and solar energetic particles so that life began, even if for only a short while? Even if life were not an issue, many aspects of plan- etary behavior are affected by the properties of the in- duced and intrinsic magnetic fields. Atmosphere is be- ing lost from Mars and Venus and in the past, from the Moon and Mercury. The moons of Jupiter interact strongly with the Jovian magnetosphere, and one moon, lo, is responsible for turning that planet into a powerful radio source. Many of these processes may have coun- terparts on an astrophysical scale. Radio sources, mag- netic flux tubes, x-ray production, shock waves, and cosmic ray acceleration may all have analogues in the more accessible terrestrial magnetosphere and provide some ground truth for astrophysical processes. Paradoxically, the accessibility of space plasma is also important for laboratory plasmas. Laboratory de- vices often cannot be probed because of their size or operating conditions. When an analogous process oc- curs in space, an instrument can be placed in the space plasma without disrupting the system, whereas in the laboratory an instrument might interfere with the pro- cess, if it is able to operate at all. Most important, space plasmas provide ground truth for computer simulations. Codes can be validated on space plasma processes and then used in situations where there is no ground truth because of the dangerous conditions involved in testing the system. SUMMARY Magnetized plasmas are an exotic state of matter in which processes occur that are outside our usual experi- THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS once. We need a combined program of observation, theory, and modeling to understand them, and we need to perform these studies under a variety of conditions. The processes that occur when a magnetized plasma flows past an obstacle are particularly important and challenging. Three important aspects of such plasmas are that the magnetic fields can act as both shields and accelerators, that they can generate and annihilate magnetic fields, storing and releasing energy in the pro- cess, and that the coupling between the various plasma regimes occurring in planetary magnetospheres is com- plex. Understanding the physics of these magneto- spheres is important to planetary scientists, to astrophysi- cists, to laboratory physicists, and to the inhabitants of this planet. 2.2 MAGNETOSPHERES AND THEIR PARTS OVERVIEW Magnetospheres consume huge amounts of power and extend over vast volumes of space. They are pow- ered by both the solar wind and internal energy sources. Jupiter, for example, has a very strong internal energy source, extracting energy from the rotation of the planet. At the other extreme, the dynamics of Earth's magneto- spheric plasma is largely driven by the solar wind. The size of a magnetosphere is determined by the balance between the pressure exerted by the magnetosphere outward and the pressure of the solar wind confining the magnetosphere. The vast extent of the Jovian magneto- sphere is due to the strong magnetic dipole moment of the planet and the strong outward centrifugal force in its dense, nearly co-rotating magnetodisk, balanced by a relatively weak solar wind. In contrast, Earth's magneto- sphere is smaller because its magnetic moment is much weaker than that of Jupiter, the solar wind pressure is stronger, and there is very little internal plasma pressure. Venus has essentially no internal magnetic field, but it does possess a thick atmosphere and a significant iono- sphere. The solar wind induces a "magnetosphere" that is not much larger than the diameter of the planet but otherwise similar in shape to Earth's magnetosphere. The solar wind is magnetized and mainly flows around planets rather than being absorbed by them, whether they have intrinsic or induced magnetospheres, and the interaction between the solar wind and plan- etary magnetospheres is quite complex. While there is a

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS direct correlation between the size of an intrinsic mag- netosphere and solar wind dynamic pressure, the mo- mentum transfer between a magnetosphere and the solar wind depends also on the strength and relative orienta- tion of the planetary and solar wind magnetic fields, the latter generally called the interplanetary magnetic field (IMP). Understanding the coupling of the variable solar wind to a planetary magnetosphere requires fundamen- tal knowledge of the properties of magnetized plasmas. The more than 40 years of research on planetary magnetospheres has yielded a wealth of information about the general properties of the solar wind-magneto- spheric i Interaction. The characteristics of Earth's mag- netosphere serve as well-documented examples of the complex interaction between the solar wind and the planet's magnetic field. As illustrated in Figure 2.4, detached from the obstacle in the solar wind is a col- lisionless bow shock that acts to slow the solar wind to subsonic speeds and partially deflect it around the mag- netosphere. Between the bow shock and the magneto- pause is the magnetosheath, where plasma undergoes further slowing and deflection around the magneto- sphere. The magnetopause is the location where the inward pressure of the solar wind is balanced by the outward pressure of the magnetosphere. On the day- side, the magnetosheath magnetic field is enhanced at the magnetopause and the density depleted. The degree to which the field is enhanced and the density depleted depends on the external solar wind conditions and the magnetic field direction. On the nightside, as illustrated in Figure 2.1 above, the solar wind flow past Earth stretches Earth's field lines into a long tail. The dynamics of this tail is very strongly correlated with the solar wind magnetic field orientation and the solar wind pressure. Inside the magnetopause, other plasma regions are formed as a result of the solar wind interaction. In general, these plasma regions also have distinct properties and are separated from each other by thin boundaries. This cellular structure is a distinct characteristic of the interaction of collisionless magnetized plasmas. At high latitudes are the polar cap, polar cusp, plasma mantle, and lobe regions. These are generally regions of open magnetic field lines (i.e., one end of the field terminates in Earth's ionosphere and the other ter- minates on the Sun) created by magnetic reconnection or by i ntercon nection of sol ar wi nd and magnetospheric magnetic fields. A relatively dense plasma sheet is formed around the neutral sheet in Earth's magnetotail. Closer to Earth, pi asmas are energized, creati ng the ring current and radiation belts (or Van Allen belts, as ~9 .~1 they were original Iy named). Final Iy "cold" plasma from Earth's ionosphere (~1000 K plasma relative to a ring current temperature of ~1 08 K) popu lates a torus near Earth, called the plasmasphere. Similar structures are seen in other planetary magnetospheres such as those of Mercury and Jupiter, but they are modified there by the presence of other sources of plasma (such as satellites and rings) and by the strength of these plasma sources and internal magnetic field relative to the surrounding solar wind conditions. Each of these regions is discussed in detail in the subsections below. Significant achievements are dis- cussed first, followed by outstanding questions. In general, the similarities and differences between Earth's magnetosphere and other planetary magnetospheres are treated in each particular subsection. However, some properties unique to the interaction of the solar wind with other planetary magnetospheres and small bodies in the solar system are discussed separately. BOW SHOCK Achievements The investigation of Earth's bow shock spans much of the history of space physics in the spaceflight era. Initially it was thought that collisionless shock waves could not exist, because the mean free path of a solar wind particle was larger than 1 AU. Later, it was realized that waves in plasmas could provide the dissipation that would have been provided by particle-particle collisions in a collisional shock. However, waves were not the complete answer. It soon became apparent that the heat- ing across a shock (dictated by the magnetobydrody- namic jump conditions across the discontinuity) could not be provided by wave dissipation above a certain critical Mach number. Above this Mach number, a frac- tion of the incident solar wind ion beam is reflected from the shock by a combination of the increased mag- netic field and the shock electric field that is along the normal in the direction to slow the ions. The reflected ions return to the solar wind. The ultimate fate of these ions depends strongly on the angle between the solar wind magnetic field and the normal to the (curved) shock surface (0bn). Later it was appreciated that these processes are present even for subcritical shocks. For In < 45 degrees, particles that reflect from the quasi-paral lel shock or that "leak" from the downstream region do not return to the shock (see Box 2.1 on dissi- pation). The backstream ion (and electron) beams found in the foreshock (see Figure 2.4) interact with the solar

60 \ \ \N /'~'~ \ ~\1~/~ ~ \ \~A ~ 'A ~~ - - - - - - - - - - - - - An- - - -~-~-~-~-~-~-r~ Aim \ \\ ~ \ \ \ ~~ ~~ ~ ~ \1 ~ _ ~ \ \ \\~N ~~ ~ ~~ — \ \ I\ \ \ \~> ~ \ ~ ~ - -1 -1 Field Line -----Streamline - 1 Foreshock Boundary FIGURE 2.4 The interaction of the solar wind and the interplan- etary magnetic field with a magnetospheric obstacle. Shaded is the foreshock region,where reflected ions cause disturbed solar wind, which is convected into the magnetosheath. Courtesy of C.T. Russell, University of California, Los Angeles. wind, producing a wide variety of plasma wave modes. The group velocity of many of these large-amplitude waves is less than the solar wind velocity, and the waves are convected back into the shock. These waves play an important role in particle acceleration, especially of the high-energy tail of the ion distribution. In particular, owing to growth and saturation of the wave modes and generation of modes that compress the magnetic field and plasma populations, the convection of the waves back into the shock creates a set of converging "mir- rors," where ions are accelerated in the first-order Fermi process. THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS Observed high-energy populations in the foreshock region agree extremely wel I with predictions from Monte Carlo simulations of this acceleration process. As ions become energized in the converging mirrors, their prob- ability of escape from the system increases. Thus, the maximum energy attained by first-order Fermi accelera- tion depends simply on the size of the interaction re- gion. At Earth, the maximum energy gain is about 150 times the original energy of the solar wind. Evidence of this scaling is seen when observations near Earth's bow shock are compared with observations at the much larger bow shock at Jupiter. Another type of acceleration can occur near the shock. Ions and electrons can reflect off the shock or escape from the downstream region near the region where the convecting solar wind magnetic field is nearly tangent to the shock interface. These particles can "surf" or drift along the shock front, in the direction of the tangential electric field seen in the shock frame, gaining (or losing) energy while staying on the convecting mag- netic field that is nearly tangent to the shock surface. This time-stationary unipolar electric field exists because an observer in the shock rest frame sees plasma and magnetic field advancing through the shock surface. Through this process, sometimes called fast Fermi accel- eration, a small fraction of the incident solar wind par- ticles can gain ~30 to 100 times their original energy. A paradox soon was realized in the electron behav- ior: namely, that the same electric potential that slowed the ions should accelerate the electrons, whereas the electrons were heated less by the shock than were the ions. The resolution to this paradox is that electrons remain tied to the magnetic field as they cross the shock and move along that field and across equipotentials im- posed by the solar wind in such a way that they see a minimal change in electric potential. Finally, the study of Earth's bow shock has led to a better understanding of the preconditioning of the plasma and magnetic field that ultimately interact with Earth's magnetosphere. This understanding goes well beyond the prediction of bulk properties of the shocked solar wind plasma, which are well reproduced in global MHD codes. Quasi-parallel and quasi-perpendicular shocks, where the IMP is parallel to and perpendicular to the shock normal, respectively, have different effects on the magnetosphere. For example, mirror-mode waves produced at the bow shock and in the magnetosheath interact with the magnetopause when the magneto- sheath in the vicinity of the Sun-Earth line is down- stream of the quasi-perpendicular shock (this is the typi- cal situation for the Parker spiral angle of the solar wind

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS 61 The development of our understanding of the mechanisms responsible for dissipation at collisionless shocks such as Earth's bow shock is an excellent example of the interplay between analytic theory, in situ observations, and computer modeling. Early in shock research, it was apparent that the structure of the shock depended strongly on the Mach number of the solar wind flow and the angle between the solar wind magnetic field and the normal to the (curved) shock surface (0bn). In particular, above a critical Mach number, resistivity alone was shown to be inadequate to provide the change from upstream to downstream that was required to satisfy the magnetohydrodynamic jump conditions across the shock. Deter- mining the dissipation mechanism for supercritical shocks had to wait for advances in space plasma instrumentation and improvement in computing facilities. In the late 1 970s and early 1 980s, these two technological advances were achieved, and it was found that the dissipa- tion at a supercritical shock occurs through reflection of a portion (~10 to 20 percent) of the incident solar wind ion beam off the shock front. For shock normal angles greater than 45° (0bn > 45°), these reflected ions execute approximately one half of a gyro-orbit in the upstream region, gain energy in the solar wind electric field, and return to the shock. Computer simulations showed that the reflection required both electric and magnetic forces at the shock and led to distinctive signatures in the magnetic field and phase space distributions of the particles near the shock.These features were conclu- sively identified in high-time-resolution, in situ observations near Earth's bow shock.Thus, by the mid 1 980s,the dissipation at Earth's quasi-perpendicular bow shock (i.e., the part of the bow shock where Ban > 45°) and other perpendicular bow shocks was reasonably well understood. However, there was a fundamental problem in applying this same dissipation mechanism to quasi-parallel shocks (i.e., the part of the bow shock where Ben < 45°) In contrast to the quasi-perpendicular shock,simple particle trajectory calculations showed that ions reflected offthe quasi-parallel shock in a uniform upstream magnetic field would not return to the shock to provide the necessary dissipa- tion. Once again, detailed computer simulations and in situ observations combined to provide an answer. The non- uniformity of the upstream magnetic field and the instability of the shock itself are critical in understanding the dissipation mechanism at this type of shock. Computer simulations showed that the quasi-parallel shock undergoes a reformation process whereby the shock front steepens and forms a structure similar to that seen at the quasi-perpendicular shock, but continues to steepen and overturn.The overturning process creates a turbulent transition between the upstream and downstream states that does not resemble a uniform shock transition. Detailed in situ observations downstream of the quasi-parallel shock showed evidence of a periodic, quasi-perpendicular-like dissipation mechanism.These periods were interspersed with a hot, isotropic downstream population consistent with a turbulent transition from the upstream to downstream states.Thus, by the beginning of the last decade, the dissipation mechanisms at planetary bow shocks (and interplanetary shocks) were understood from a theoretical perspective and confirmed from high-resolution, in situ mea- surements at Earth's bow shock. magnetic field). When this region is downstream from the quasi-paral lel shock, the magnetosheath conditions are dramatically different. Large fluctuations in the plasma beta (ratio of the perpendicular pressure of the plasma and the magnetic field) and significantly in- creased magnetic turbulence produce a very different interaction with the magnetosphere. Also, particle distri- butions produced at the shock can be observed directly in the magnetosphere. A case in point is the recently discovered energetic ion population in Earth's magneto- spheric cusps. This population has all the characteristics and solar wind dependencies of the energetic ion popu- lation created by first-order Fermi acceleration at Earth's bow shock. Outstanding Question HowAre Shocks Modified by Multiple Components? The development of a clear understanding of dissi- pation at Earth's bow shock has led to the possibility of understanding other types of collisionless shocks. In par- ticular, Earth's bow shock represents a relatively clean example of a planetary bow shock where the supersonic solar wind is decelerated in response to the presence of Earth's magnetosphere. Even in this relatively clean ex- ample, the modification of the quasi-parallel shock due to the presence of a small (1 percent) population of "reflected" ions is profound. In other collisionless shocks, additional modifications occur because of the

62 presence of additional plasma populations. For example, at comets, the outgassing of the comet and subsequent ionization of its extended atmosphere creates a popula- tion of ions that are picked up by the solar wind. In fact, mass loading is so strong at comets that the accompany- ing deceleration produces an obstacle to the solar wind, which produces the bow shock. Plasma observations from spacecraft flybys of com- ets have not been sufficient to determine completely the nature of the changes in the shock due to the addition of heavy ions. These in situ investigations are important because other shocks of astrophysical interest (e.g., the hel iospheric termination shock and supernova shocks) probably have significant effects attributable to the presence of secondary components such as high-energy particles (e.g., galactic cosmic rays). From the recent experience with Earth's bow shock, understanding the dissipation mechanisms at cometary bow shocks will require a strong interplay between computer simula- tions and high-resolution, in situ measurements. MAGNETOSHEATH Achievements The magnetosheath is the region of shocked solar wind that is bounded by the thin boundaries of the bow shock on the upstream (or sunward) side and the mag- netopause on the downstream (or earthward) side. Be- cause the solar wind is supersonic, moving faster than the speed of the compressional wave in a plasma, the shock is the first place at which the solar wind is decel- erated, with the possible exception of interactions with backstreaming ions, discussed in the preceding section. Once downstream from the shock in the subsonic mag- netosheath, the plasma can be further slowed and de- flected continuously by pressure forces. Eventually the plasma reaccelerates as it moves past Earth, reaching speeds comparable to those of the solar wind. Magnetobydrodynamics dictates that there are two other low-frequency waves that can stand in the mag- netosheath downstream from the fast mode shock and upstream of the magnetopause. These waves are the intermediate wave, which rotates the field and the plasma flow, and the slow mode, which can increase the field strength whi le decreasing the density. Currently, one of the controversies is how sharp these transitions must be. For example, a slow mode transition is pre- dicted and possibly observed just upstream of the mag- netopause, but other interpretations of and reasons for the absence of this effect have been offered. This is an THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS active area of research, and it is important because it involves understanding the basic physics of a magne- tized plasma. What is more, this mechanism determines how strongly the solar wind will interact with the mag- netosphere. The processes occurring at the inner edge of the magnetosheath, the magnetopause, are affected by the modification in the plasma as it moves through the magnetosheath. One effect is the formation of a plasma depletion layer adjacent to the boundary. Accompany- ing this reduction in the plasma density is an increase in the magnetic field strength. When the IMP has a south- ward-directed component (at Earth), reconnection in the subsolar region is more prevalent than when the IMP has a northward component and the plasma depletion is weaker. The understanding of the process of plasma depletion and piling up of the field has resulted in a successful prediction of the temperature anisotropy of the ions in the magnetosheath and plasma depletion layer (see Box 2.21. The existence of a depletion layer has important consequences for reconnection at the magnetopause. These consequences are discussed in the next section, "Magnetopause, Cusp, Boundary Layers." The magnetosheaths of other planets have proper- ties similar to Earth's magnetosheath. In particular, simi- lar magnetic field turbulence is observed, as are similar plasma transitions. The magnetosheaths of active com- ets are interesting in that a strong region of piled up magnetic flux is formed outside the contact surface (the region where the solar wind magnetic field is excluded). This region is similar to the pileup of magnetic field at Earth's magnetopause. Also, comets and some planets can have different interactions in their magnetosheaths. At Earth, relatively little plasma escapes from the mag- netosphere and enters the magnetosheath. This plasma usually consists of a relatively high-energy population having a density that is significantly less than the density of the shocked solar wind plasma. Other planets and comets have stronger internal sources. These produce significant particle populations in the magnetosheath, which affects the wave-particle interactions in the re- gion, especial Iy at comets. The mass loadi ng produced by these ions also slows the flow and further increases the size of the i educed magnetosphere. Outstanding Questions WhatAre the Specific Wave Mode Transitions in the Earth's Magnetosheath and How Sharp Are They? As mentioned in the discussion above, the MHD description of the magnetosheath suggests that there

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS 63 Spacecraft observations in Earth's magnetosheath show a distinct inverse correlation between the ion temperature anisotropy and the ratio of the perpendicular plasma pressure to the magnetic field pressure (plasma beta).Theoretical models of the magnetosheath reproduce this inverse correlation through wave-particle interactions. As plasma convects toward the magnetopause, the magnetic field increases, the perpendicular temperature of the shocked solar wind ions increases, and, because of losses along the magnetic field, the ion parallel temperature decreases. The increase in the magnetic field is accompanied by a decrease in the shocked solar wind density, which results in a reduction of the plasma beta as the shocked solar wind approaches the magnetopause. The increase in the temperature anisotropy as the magnetopause is approached cannot continue unabated. Above a certain critical anisotropy,which depends on the plasma beta,the electromagnetic ion cyclotron instability is excited in the convecting plasma. Like all wave instabilities in a plasma,the waves interact with the particles and reduce the free-energy source of the instability.Thus,the temperature anisotropy decreases and the instability is quenched. As the plasma contin- ues to approach the magnetopause,the anisotropy grows again until the ion cyclotron instability is once again excited.The net result is a temperature anisotropy that is maintained near the threshold of the ion cyclotron instability.This threshold depends inversely on plasma beta, so that the observed anisotropy-beta relation in the magnetosheath is reproduced by application of wave-particle interactions. This understanding is not incorporated in global MHD models but relies on kinetic and hybrid (massless electron) approaches. should be specific wave mode transitions between the fast mode transition at the shock and the magnetopause. It is still an open question how sharp (and indeed if) these transitions occur. The understanding of these fea- tures in the magnetosheath will lead to a better under- standing of the plasma that interacts with the magneto- sphere at the magnetopause. What Are the Plasma and Field Conditions Adjacent to the Magnetopause? Surprisingly, an accurate description of the plasma and magnetic field parameters in the magnetosheath adjacent to the magnetopause is still lacking. Current state-of-the-art studies of the solar wind-magnetosphere interaction use either gas dynamic modeling (e.g., the Spreiter and Stahara gas dynamic model) or isotropic, single-fluid MHD. As a result, the total magnetic field, the ion and electron temperatures, and the temperature anisotropies (and therefore the division of plasma pres- sure into perpendicular and parallel components) have not been described accurately on a global scale. As will be seen in the next section, these input parameters are critical for understanding the most important plasma transfer process at the magnetopause, magnetic recon- nection. Because the boundary is dynamic and has an a priori unknown geometry, including nonplanar struc- ture, resolving this issue will require multipoint mea- surements of the plasma and magnetic field in the magnetosheath as well as three-dimensional, hybrid computer models to investigate the physical processes leading to the observed features discussed in Box 2.2. MAGNETOPAUSE, CUSP, BOUNDARY LAYERS Achievements The magnetopause is where the external plasma and magnetic field pressure of the magnetosheath meet the internal plasma and magnetic field pressure of the mag- netosphere, establishing a pressure balance. It is usually considered to be the outer boundary of the magneto- sphere. The structure of Earth's magnetopause is com- plicated because the plasmas on both sides of the dis- continuity are magnetized, so that when the fields on opposite sides of the boundary are nearly antiparallel they can reconnect. This connection spoils the simple topological distinction between solar wind and mag- netospheric plasma by creating a region that contains both plasmas on the same magnetic field lines. Under these circumstances it is best to think of the magneto- pause as a region rather than a thin discontinuity. For typical solar wind conditions, the distance from Earth's center to the magnetopause along the Earth-Sun line, called the subsolar distance, is about 1 0 times the radius of Earth (1 RE = 6,371 km). The shape of this boundary can be approximated by an ellipsoid of revo- lution about the Earth-Sun line, with the magnetopause at the terminator about 50 percent further from Earth than it is at the subsolar point. In the magnetotail, this approximation is not accurate. The magnetotail contin- ues to expand in cross section and extends many hun- dreds of Earth radii in the direction away from the Sun. Since the magnetopause is in pressure balance, changes in the internal or external pressure cause the

64 location and shape of this boundary to change. The solar wind dynamic pressure (that is, the solar wind mass density times the square of the solar wind velocity) is far from constant. This dynamic pressure is converted to plasma thermal pressure and magnetic field pressure in the magnetosheath. Thus, variations in the solar wind dynamic pressure cause the magnetopause to move to- ward and away from Earth. The subsolar distance can move in to about half its nominal distance for extremely high solar wind pressure and can move out to about twice its nominal distance for extremely low solar wind pressure. The size of planetary magnetospheres varies greatly in absolute and relative scales. At Mercury the magnetosphere's nose does not extend much above the planet's surface on the dayside. In contrast, at Jupiter many of the planet's moons orbit inside the magneto- sphere. If the plasma and magnetic field in the magneto- sheath and the magnetopause interacted only at this basic level, then the physics of the interaction would be already solved. However, the major difference between THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS this simple interaction and that at Earth's magnetopause is that it neglects the external magnetic field in the magnetosheath. Although the magnetic field in the mag- netosheath is weaker than its magnetospheric counter- part, it is the presence of this magnetic field that causes most of the interesting phenomena associated with Earth's magnetopause. In 1961 J.W. Dungey suggested that the solar wind and terrestrial magnetic fields would interconnect at a neutral point when the magnetospheric and solar wind magnetic fields were nearly antiparallel (see Box 2.3~. Under certain assumptions, the change in plasma pa- rameters at the magnetopause can be predicted, allow- ing the reconnection hypothesis to be tested. Further- more, the kinetic treatment of the magnetopause, in which particle motions are followed, leads to specific predictions for the ion and electron distributions that can be observed in the various magnetopause layers. Spacecraft- and ground-based observations in the last decade have firmly established that reconnection is the Although predicted by J.W. Dungey in 1961, the details of magnetic reconnection and its possible application to the magnetopause were not worked out until several years later. As discussed above, reconnection interconnects magnetized plasmas on different sides of a boundary, often leading to accelerated flows.This signature of magnetic interconnection accompanied by fast flows has become the major indicator of the presence of reconnection. Predictions for antisotropic MHD were developed in 1970 under the assumption that the magnetopause is locally a one-dimensional rotational discontinuity whose properties remain steady on time scales long compared with the transfer of a single-fluid plasma element across the discontinuity. Under these assumptions, the MHD equations yield inequalities at the magnetopause that are related to the reconnection rate. Specifically the normal component of the magnetic field at the magnetopause, Bn, is not zero across a rotational discontinuity but is zero across a tangential discontinuity across which the fields on the two sides are not connected.While difficult to quantify,this nonzero normal component has been observed at the magneto- pause. Spacecraft observations (initially two-dimensional plasma observations in the 1 970s and then three-dimensional observations in the mid-1980s) became detailed enough to provide additional observational verification of predictions from single-fluid MHD theory. Even with the improved observations, some additional inequalities such as the normal component of the velocity and the tangential component of the electric field are exceedingly difficult to measure because they are small compared with the motion of the boundary in response to the ever-changing solar wind. The existence of a normal component of the magnetic field across the magnetopause requires the magnetized plasma on both sides of the discontinuity to flow together.This leads to the existence of a reference frame, called the de Hoffmann-Teller (HT) reference frame, which slides along the magnetopause with the tangential velocity of the intercon- nected field lines. By itself,the existence of an HTframe is a necessary (but not a sufficient) condition for reconnection.Two important relationships develop out of the tangential Maxwell stress conditions and the existence of an HT frame at the boundary. These relationships also represent necessary, but not sufficient, conditions for the existence of magnetic re- connection.The first is related to mass conservation across the magnetopause and is difficult to test without high-resolu- tion mass-resolved measurements at the discontinuity.The second, often referred to as the Walen relation, indicates that plasma on either side of the discontinuity flows at the local Alfven speed in the HT frame. Spacecraft observations have demonstrated that ion (and more accurately electron) flows at the magnetopause often have this Alfvenic flow property. The verification of these predictions and those at other scale lengths firmly establishes reconnection as an important transfer process at the magnetopause. . . .. . . . . . .

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS primary process for the transfer of mass, energy, and momentum across the magnetopause and intimately controls the properties of the polar cusps (see Box 2.4~. When magnetic reconnection occurs for directly southward interplanetary magnetic fields (at Earth), the magnetosheath and magnetospheric magnetic field I i nes interconnect in a relatively small region (often called the diffusion region) near the subsolar point. Because the diffusion region is very small and (possibly) moving, the probability that a spacecraft will cross through the diffu- sion region is thought to be small. The understanding of the physics of this region is critical to understanding how reconnection works, and the understanding of re- connection is important to astrophysics and solar phys- ics as well as to planetary magnetospheres. Outstanding Questions What Is the Global Reconnection Rate at the Magnetopause and How Does It Change with IMF or Solar Wind Conditions? The MHD tests of magnetic reconnection at the magnetopause fall into two separate categories. In- equalities such as the nonzero normal component to the magnetic field relate to the reconnection rate. Tests of the magnitude of the tangential flow velocity, such as the Walen test, are independent of the reconnection rate. Unfortunately, because the reconnection rate is very low, quantitative measure of this rate is exceed- ingly difficult using, for example, the normal compo- nent to the magnetic field or the normal component to 65 the flow velocity. Thus, the inflow rate is not well known. The total extent of the reconnection region is also not well known. Typically, single spacecraft observe recon- nection features at a single point on the magnetopause or two or more spacecraft separated by a small distance (when compared with the total magnetopause extent) observe reconnection signatures. Ground-based radar observations are very important in this regard, especially the SuperDARN radar network. Both the rate and the total extent of the reconnection region are required to determine the total inflow rate into the magnetosphere. Without a measure of this rate, it is difficult to assess the effectiveness of reconnection in providing the mass, energy, and momentum transfer into the magnetosphere. Certainly this transfer is a func- tion of the external solar wind conditions, especially the IMF orientation. It is also a function of geometry. Re- connection generally occurs in a three-dimensional geometry while theoretical treatments are generally done in two dimensions. What Is the Interplay Between the Microscale and Mesoscale Aspects of Reconnection? Is the Dissipation Driven by External Boundary Conditions or Internal Microscale Instabilities? Despite the small size of the region where magnetic field lines interconnect (the diffusion region), it has glo- bal consequences. The diffusion region remains one of the last unexplored regions of the magnetopause, and its study is a matter of very high priority. It consists of tran- sitions on several scale lengths. Two of the more impor- tant transitions are the regions where the ions decouple Following his prediction of magnetic reconnection at Earth's subsolar magnetopause during periods when the IMF had a southward (or negative Bz) component,J.W. Dungey predicted that there would be reconnection at the high-latitude magnetopause during periods of northward IMF. Since magnetic reconnection provides a means to interconnect solar wind magnetic field lines with those in the magnetosphere, these two predictions indicate that there could be a nearly continuous connection between the magnetosphere and Earth's bow shock. Nowhere is this connection more apparent than in Earth's magnetospheric cusps.These high-latitude regions exhibit direct entry of solar wind plasma. Recent modeling and detailed comparison of these models with observations have shown that the properties of the cusp are consistent with magnetic reconnection at the magnetopause. New observations made in the last decade showed the presence of an energetic ion component of solar wind origin that had gone undetected. It was first suggested that this energetic ion component was accelerated out of the magneto- sheath ion population in the cusp.While some such acceleration may take place, analysis of the recent observations and comparisons with global MHD simulations indicate that this energetic ion component is a by-product of the nearly con- tinuous connection between the magnetospheric cusps and the solar wind. Solar wind ions energized at Earth's quasi- parallel bow shock have direct access to the cusp along reconnected field lines.

66 from the magnetic field and the much smaller region where the electrons decouple. Ion decoupling, in a lo- cation Cal led the Hal I region, has recently been reported for reconnection in Earth's magnetotail. The lack of ob- servations and the difficulty in theory to deal with very disparate scale lengths has led to some ambiguity about what controls reconnection. Certainly boundary condi- tions must play an important role in modulating the rate of reconnection, but so too the conditions at the re- connection point must be important, perhaps control- ling where reconnection takes place. This control in turn will affect the transfer of magnetic flux from closed to open field regions. Thus the determination of the role of microphysics in reconnection is critical to understand- ing of magnetospheric dynamics. Is Reconnection Patchy or Quasi-continuous, Inherently Unstable or Quasi-static? Does It Occur When Magnetic Fields Are Exactly Antiparallel or When Only One Component Is Oppositely Directed? The preceding question dealt with the "why" of reconnection. These questions relate to the "when, where, and how much" of reconnection. However, they also have important implications for the "why" of re- connection. All of them remain unanswered because of one fundamental limitation of previous observations and several fundamental problems with the reconnection process and observations at the magnetopause in gen- eral. Previous measurements at the magnetopause were mostly from single spacecraft or from two or more space- craft separated by distances that were small compared with the possible scale sizes of patchy reconnection. Patchy reconnection could be occurring on scale sizes as small as a few thousand kilometers or as large as several Earth radii. Field-line convection away from the reconnection site can give the appearance of a large reconnection region when in fact the neutral I ine where the diffusion is occurring is relatively short. Thus, it is not sufficient to have large spacecraft separations; they must be separated in the appropriate direction. Fundamental problems with the reconnection pro- cess and with observations at the magnetopause in gen- eral include the difficulty of measuring the reconnection rate using in situ measurements at the magnetopause, the constant motion of the magnetopause so that obser- vations are limited to brief crossings of the boundary, changes in the location of reconnection that follow the variable IMP direction so that spacecraft positions at the boundary may be ideally suited for only a narrow range of IMP orientations, and the large variety of scales (from electron to ion to current sheet thicknesses) associated THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS with reconnection, ranging from <1 km to many RE. These limitations and problems cannot be overcome by any mission consisting of a single spacecraft. Multi- spacecraft missions carrying high-time-resolution instru- ments are needed. MAGNETOTAIL Achievements The extended magnetotail is the direct manifesta- tion of the interaction between Earth's magnetic field and the solar wind (a magnetotail is similar to cometary tails, whose study led to the discovery of the solar wind). The magnetotail is also relevant because it plays a major role in the dynamics of the magnetosphere, acting as an energy storage region and governing the sudden release of this energy in magnetospheric substorms. Progress in recent decades has led to a much more detailed under- standing of activity in the tail and its connection with Earth. Earth's magnetotail exhibits various modes of activ- ity, including not only substorms but also pseudosub- storm onsets ("pseudobreakups") and steady magneto- spheric convection (SMC) events. All of these may be accompanied by short-duration fast plasma flows, so- called bursty bulk flows (BBFs). Bursty bulk plasma flows in the plasma sheet (see Box 2.5) are fast flows typically lasting 10 minutes and having individual peaks of about a minute. Pseudobreakups are smal I activations, observed in both the magnetotail and the ionosphere prior to substorms, that do not lead to the global reconfiguration of the magnetotail and the expansion of the auroral oval as do full substorms. SMC events are characterized by overall stability under steady, driving solar wind conditions; numerous transient activations; and an absence of substorms. Even in the absence of substorms, the tail plasma sheet is found to be highly variable, giving the appear- ance of a turbulent rather than a laminar flow state. Since the maximum scale size, the tail diameter, is only about two decades larger than the ion gyroradius, how- ever, the MHD turbulence in the tail is far from homoge- neous. Since the near-Earth plasma sheet is a source of other populations (e.g., the ring current and the radia- tion belts), its variabi I ity affects those popu rations as well. The overall evolution of the magnetotail during a substorm is well understood. It consists of a growth phase, initiated by southward IMP, during which mag- netic flux and energy are transported from the front side

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS 67 The introduction of the open magnetospheric convection model was one of the great conceptual inventions of the early 1960s. In this paradigm, magnetospheric convection is driven by dayside reconnection when the interplanetary magneticfield (IMP) has a southward component.Subsequently, interconnected field lines are dragged into the tail,where they again undergo reconnection, thereby trapping plasma on closed field lines. Plasma and magnetic flux are then transported around Earth to the dayside,where they again can undergo reconnection with the solar wind, in a seemingly steady circulation.This paradigm still governs our basic understanding of magnetospheric circulation today. However, it is particularly the steady-state aspect of this picture that has come under scrutiny, because of inconsistencies both ob- servationally and theoretically. In a steady convection state, the electric field should be a potential field. In ideal MHD, with negligible parallel electric field, this potential is constant along magnetic field lines. However, average electric fields in the tail plasma sheet are considerably smaller than those obtained by mapping the ionospheric potential to the tail. Furthermore, the average tail configuration is inconsistent with adiabatic, i.e., entropy-conserving transport from the tail toward Earth. Entropy conser- vation would lead to pressure increases that by far exceed the pressure necessary to balance the observed magnetic forces. The solution to these discrepancies lies in the fact that the tail hardly ever assumes a steady state, and that entropy conservation on closed magnetic flux tubes is sporadically violated by the severance of plasmoids via magnetic re- connection.The manifestation of these features is that fast plasma flows in the tail plasma sheet, and correspondingly high electric fields, occur only in bursts of short (~10 mint duration with spikes of even shorter periods (~1 mint. Despite their short duration and relatively sparse occurrence, these bursty bulk flows play the most important role in the transport of magnetic flux and energy in the tail. Although the incidence of bursty bulk flows is generally correlated with geomagnetic activity, their occurrence is not confined to a particular substorm phase or even to substorms alone.They are frequently observed during steady magneto- spheric convection events, characterized by the absence of substorms under steady solar wind driving conditions.They are apparently highly correlated with auroral intensifications. of the magnetosphere to the tail, where they become temporarily stored in increasingly stretched and en- hanced magnetic fields. This phase is followed by a release phase, during which some inner portion of the tail collapses toward a more dipolelike field, while an outer portion of the plasma sheet, a "plasmoid," be- comes detached via magnetic reconnection and ejected antisunward. Tail observations have shown that this re- connection site on average forms between 20 and 30 RE down the tail (see Box 2.6~. A recovery phase restores the tail to its original state. However, not all accept this model, wh ich predicts that the substorm process begi ns in the near tail, as illustrated in Figure 2.5. Some predict that the onset begins at lower latitudes and moves tai Iward. The fi rst two phases are wel I understood and have been successfully modeled on large scales, including the acceleration of particles (see Box 2.7~. The processes that govern the transition from one phase to another are less well understood. Various observations, as well as plasma simulations using a variety of approaches, have demonstrated the important role of the formation of a thin current sheet in the near tail, embedded in the wider plasma sheet, prior to the substorm release phase. As at the magnetopause, the development of strong gradients with small characteristic scales is a necessary condition to break the frozen-in flux condition of ideal MHD, en- abling current disruption and magnetic reconnection. Thus, a thin sheet with sufficiently small scales is a cru- cial step toward instability. Advances in plasma simulations have also shed more light on the physics of reconnection, at least under laminar conditions. The simulations suggest a character- istic structure with threefold-embedded regions. The in- nermost region is governed by nonadiabatic electrons, wh ich provide the dissipation mechanism. Th is region is surrounded by a Hal I region, where electrons are coupled to the magnetic field but ions are still un- coupled. The outermost region then is governed by fluid- like behavior, where both particle species are coupled to the field. Although reconnection underlies many of the pro- cesses that govern both the tail and the magnetopause, it is not sufficient to study just one or the other of these

68 THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS The steady magnetospheric convection paradigm for southward IMF involves two magnetic reconnection sites,at the dayside magnetopause and in the distant tail. The great conceptual invention of the 1970s was the realization that a further reconnection site should form sporadically in the near tail at a near-Earth neutral line. This solves not only the entropy and mass transport prob- lem but also provides an expla- nation for the generation of fast plasma flows. Thorough statistical analyses on the basis of Geotail ob- servations in the past decade have confirmed the existence of these neutral lines (reconnection sites) and provided their average loca- tions. On the basis of the occurrence of fast earthward or tailward flows related to substorm onsets, it is found that the near-Earth reconnec- tion site typically forms at 20-30 RE distance down the tail. In contrast, the location of the distant reconnec- tion site or neutral line appears to be much more variable. Its location apparently varies,typical Iy from ~60 RE to distances beyond 200 RE Occa- sionally it may also be located inside 60 RE Figure 2.6.1 illustrates the relative magnetic topology of the two neutral points and how the magnetic flux in the tail might re- spond to southward and northward turnings. ~ ~ RD ONPS ~)~ ~ P NL R .. . R.. Me ~ ~ ~ N · ~ . Cal (L Z . . . . N . . . . , . . . \ · / · · · \ , . . . \ , . . . ,, ~ . . . . . ., ., . . . . ~Thin, , ~ Exp ~ . . ... . . ... . . . . . . . . ~ . ~ ~ ~ /' ~ ~ O . . . . . ~ . . .. . . ~ . . . ~ . . in . , \, . . .. . ~ ~ ~ 'A Time FIGURE 2.6.1 Two neutral point models of the tail showing how the magnetic flux in different regions of the tail responds to variations in the IMF and the initiation and termination of reconnection.The amount · c of magnetic flux in the closed dayside magnetosphere, ODay, the open lobe of the tail,~Obe,the closed near-Earth plasma sheet, ONPS, the plasmoid, ~PM, and the distant plasma sheet, ODPS, are all modu- lated by reconnection at the magneto- pause and at the near and distant reconnection points at rates M, RN'and RD, respectively, and by the return convective flow with rate C.These rates change when - the IMF turns southward to initiate trans- fer from the dayside to the lobe at time S; at the time of onset of plasmoid formation, P. when the IMF turns northward, N; and when reconnection reaches the open field lines of the lobe L. Courtesy of C.T. Russell, University of California, Los Angeles. regions. First, the plasma conditions are different at the two locations; second, we need to learn how recon- nection produces the observed dynamics of the magne- topause and the tai 1. For example, at the magnetopause the interaction geometry produces a current layer across which the magnetic field is generally not antiparallel, and there is no interconnecting magnetic component. In contrast, in the tail the fields in the two lobes are nearly antiparallel but are generally linked by a strong inter- connecting component. At the magnetopause, dynamic behavior occurs when a normal component arises. In the tail, dynamics occurs when the normal component reaches a very small value. Outstanding Questions Despite the many advances in understanding the magnetotail and its dynamics, major questions remain, such as the cause of substorm onset, and new questions have come up from new discoveries such as BBFs. What Causes the Onset of Terrestrial Magnetotail Activity and, Particularly, Substorms? This is one of the oldest questions. However, the competing views of the onset sequence now differ only in the timing of events and only by one or a few min- utes. Nonetheless, since the processes that occur within

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS (I Substorm Current Wedge (I) Braking and Dawnward Current <: ~ 5\ ~ OUts;e In (I) High-Speed Flow ~ = (I) Substorm Current Wedge ~ ~~ 9~ :~ '' (I) Rarefaction Wave (I) NENP Duskward Current Disrupted Inside~0ut (A NENP FIGURE 2.5 (top) The near-Earth neutral point (NENP) model predicts that the substorm begins with reconnection in the near tail. (bottom) In contrast, the current disruption model predicts that reconnection begins in the region around 8 RE and moves tailward. Both models have currents deep in the magnetosphere and reconnection in the tail but differ on the relative timing of events. Courtesy of H.E. Spence, Boston University. this time frame involve the microphysics of the onset, they are atthe core of an understanding of how collision- less dynamical processes are initiated. Solution of the controversy requires high-temporal-resolution, multi- point measurements, both in space and on the ground, 69 and a deeper understanding of the reconnection pro- cess. What Causes the Localization of Activity, Such as Reconnection, Bursty Bulk Flows, Pseudo-onsets, and, Similarly, Flux Transfer Events at the Magnetopause? Reconnection is thought to occur where the mag- netic field configuration resembles an X. If two field lines merge at a point, then the reconnection location can be referred to as a neutral point. If the merging of two field lines extends some distance across the tail, it can be referred to as a neutral line, but in this case the field strength might not be zero along its entire length. Whi le statistical studies of tai I data have narrowed down the average location where the near-Earth neutral line forms, its cross-tail extent is much less well known. In addition, as three-dimensional plasma simulations show, the extent of an active reconnection site can be much smaller than the extent of a neutral line per se. Further- more, how do we identify a neutral line and a recon- nection site in general cases when the neutral line is not strictly "neutral" anymore? Is there a "guide field" paral- lel to the neutral line? Similar problems exist for mecha- nisms that do not involve reconnection. In the case of BBFs in the tail, serendipitous multi- soacecraft observations have shown that large differ- ences in flow activity are often observed even when the spacecraft are nearly collocated. Estimates of the amount of magnetic flux propagated earthward during the sub- storm sequence compared with the amount of flux con- tained in a BBF also require that the cross-tai I dimension of the BBF be very small. Again, a constellation of glo- bally distributed spacecraft (Magnetospheric Constella- Rapid increases of energetic ion and electron fluxes in the inner magnetotail, observed extensively at geosynchronous orbit, are an intrinsic part of the substorm process.The breakdown of adiabatic particle behavior at a reconnection site has for a long time suggested the near-Earth neutral line as a potential acceleration site to generate the particles with energies of tens to hundreds of keV that constitute these energetic particle injections. However, the successful identification of the average location of the reconnection site near 20-30 RE downtail has made this interpretation inconsistent with observa- tions of dispersionless injections at geosynchronous orbit. On the other hand, the fact that flux increases are typically limited to the energy range of tens to hundreds of keV seemed to rule out an adiabatic heating mechanism such as betatron acceleration.Various simulations now have solved this dilemma by demonstrating that the induced electric field associated with the collapsing inner tail is responsible for the major part of substorm acceleration. Reconnection is the process that enables a collapse, but acceleration at the reconnection site is not a major contributor to the enhanced energetic particle fluxes in the inner tail.The apparent nonadiabatic effects are explained by the finite cross-tail extent of the acceleration region. Particles at higher energies drift too fast across the tail through this region to participate in the collapse, so that they do not experience the associated betatron or Fermi acceleration.

70 tion) has been proposed to help answer this outstanding question. What Causes Quenching of Reconnection and Other Activity? There is evidence that reconnection, as the likely cause of fast flows, sometimes ceases before lobe field lines become involved. What is the cause for this cessa- tion? When reconnection proceeds to the lobes, it ap- parently does not involve the entire lobe flux. Why is this, and when does it cease in this case? An analogous question exists at the magnetopause but may be more addressable in the tail, where the plasma is more sta- tionary, at least initially. What Is the Role of Boundary Conditions, such as the IMF North-South Direction, and What Are Some Possible External Triggers of Activity? There is plausible evidence that substorms are trig- gered by a northward shift of the IMF in perhaps half of the observed cases, with plausible evidence also that at least some substorms are not triggered externally. How does the trigger work, and why does it sometimes work and at other times not? What Is the Relationship Between Near/Midrail Reconnection on the One Hand and Substorm Onset and Substorm Auroral Features on the Other? Currently there is debate on the causal relationship between processes in the midtail and the near-geosyn- chronous orbit region during substorm onset. The de- bate centers on what role midtail reconnection plays in the near-Earth reconfiguration at substorm onset. Does midtail reconnection drive substorm onset behavior in the near-Earth region or is it a consequence of it? As mentioned above, the relevant time period covers only a few m i n uses, but an u n de rstan d i n g of th e p recesses within this period is crucial for understanding collision- less plasma instability. How Do the Mid- and Distant Tail Map to the Planet Magnetically and Dynamically? Empirical and global field models provide general qualitative mapping between the various magnetotail regions and their ionospheric counterparts. But highly variable, large-scale current systems render mappings very uncertain except in a statistical sense. More impor- tantly, a number of observations suggest that ionospheric and magnetospheric flows are not always well coupled, but we do not know the extent of this decoupling or its THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS causes or consequences. What can ionospheric mea- surements really tell us about conditions in the mag- netosphere? What Determines the Terrestrial Plasma Sheet Density? What Is the Role of the ionospheric and Plasmaspheric Plasma in Populating Earth's Plasma Sheet? How Does the Solar Wind Populate the Plasma Sheet? The plasma sheet is populated by plasmas originat- ing from various sources: the solar wind and Earth's ionosphere and plasmasphere. Changes in plasma com- position after large geomagnetic storms show that Earth's ionosphere can contribute significant mass to the plasma sheet. However, the relative contribution of the iono- sphere to the plasma sheet is not known, especially how it varies with geomagnetic activity and with radial dis- tance. There are also outstanding questions about other possible pathways from the inner magnetosphere to the plasma sheet, such as through dayside reconnection and convection back into the tail. INNER MAGNETOSPHERE Achievements Geomagnetic storms are the largest manifestation of energy coupling between the solar wind and magneto- sphere and are directly driven by solar wind transients that have large solar wind dynamic pressure and IMF Bz south magn itudes. I ntense ring currents are formed in the wake of these disturbances and are created by strong and variable convection, often reaching deep into the inner magnetosphere. The partial ring current (the cur- rent generated by energetic particles on open drift tra- jectories) has been recognized as the main contributor to the main phase ring current. Recently, efforts to make hybrid simulations by cou- pling global MHD codes with kinetic models have ad- vanced our ability to study magnetic storms and sub- storms in the midtail and near-Earth plasma sheet. This allows analyzing the behavior of non-Maxwellian distri- bution functions that are observed with i n situ space- craft and provides clues to the source mechanism and location for acceleration of energetic particles. Empi rical magnetic field and magnetospheric speci- fication models that include observations made in the inner magnetosphere over a wide range of geomagnetic conditions have significantly improved the ability to describe the configuration of the magnetosphere during geomagnetically disturbed intervals. These models are aiding in the understanding of the relevant scale sizes of

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS magnetospheric structure and contributing to the devel- opment of future multiprobe missions. The energetic ions (also known as the ring current) present one aspect of the particle energization. The strong, storm-associated, penetrating electric fields drive plasma sheet ions deep into the inner regions and to low L values during the main phase of magnetic storms. The composition of the ring current that is, the partitioning of the energy between the different species and, ulti- mately, different plasma sources has been an intense topic of discussion, research, and modeling during the past decade. The models have reached a high level of maturity, and their ability to characterize the ring cur- rent is now mostly limited by our inability to specify the phase-space distributions in the source region, the near- Earth plasma sheet. They are also limited by the inad- equacy of current electric and magnetic field models for the inner magnetosphere. Only with the Combined Release and Radiation Effects Satellite (CARES) and Polar observations from this past decade has it become clear that the electric fields penetrate very deep into the inner magnetosphere during storms, even to L values less than 2.5. The local time and radial distributions of the fields are not well known because the measurements have been single-point. The spatial picture we have of the equatorial electric fields and their variability is statisti- cal. The empirical electric field models in use today are based on low-altitude, polar-orbiting satellite and ground-based observations. As such, they do not cover the inner magnetosphere well. They also do not account for potential drops between the equator and the iono- sphere that exist during disturbed periods, nor do they include the highly time-dependent and strong inductive electric fields. What is needed is to determine the spatial/ temporal structure of the electric and magnetic fields throughout the inner magnetosphere at or near the mag- netic equator during magnetic storms and substorms. This knowledge is necessary if we are ever to under- stand the transport, energization, entrapment, and flow of energy into and through the inner magnetosphere. The energetic electrons that make up the radiation belts (electron ri ng current) are also often accelerated and transported deep into the inner magnetosphere during sudden impulses, as illustrated by the simulation in Figure 2.6. In addition, they can be accelerated by mag- ~L is the dipolar shell parameter, defined as L = RE/COS2(M), where RE is the number of Earth radii and M IS the geomagnetic (dipolar) lati- tude. For an explanation of L value, see <http://pluto.space.swri.edu/ I MAG E/g I ossary/u n its. htm 1>. 71 FIGURE 2.6 Simulation of electron acceleration in the magneto- sphere during a shock compression event. Plasma pressure shown by color contours. Also shown is a distorted ring of ener- getic electrons. Courtesy of M.K. Hudson, Dartmouth College. netic activity. It has been known for some time that increases in the flux of high-energy electrons are associ- ated with high-speed solar wind streams. Recently it has been determined that the high-speed solar wind must be accompanied by IMP Bz < 0 for the enhancements. Yet those conditions do not guarantee that the electron en- hancements occur (i.e., they are necessary but not suffi- cient conditions). The exact mechanisms that control the processes and the physics involved in the electron acceleration are not understood at this time. Several models have been proposed, from the classic radial dif- fusion picture, driven by stochastic electric and mag- netic field fluctuations, to shock resonance acceleration, recirculation models, and ULF wave-electron interac- tions. Again, the major impediment to solving the elec- tron acceleration and transport problem is the paucity of critical electron observations and poor knowledge of the background magnetic and electric fields during the acceleration events. The current state-of-the-art radiation belt models now include fairly realistic inner magnetospheric plasma populations, including the dense, cold plasmaspheric plasma. The plasmasphere is an important region for the generation of waves and the location for significant ra- diation belt loss processes.

72 Outstanding Questions Which Processes Energize Ring Current and Radiation Belt Particles? There are significant differences in the intensity of the ring current and the flux of the radiation belts from storm to storm. Which processes are involved in the generation of the two plasma populations and what dif- ferences in solar wind conditions determine the efficacy of the processes? What Is the Nature of the Source Population for Inner Magnetosphere Electrons (and lonsJ Before and During Events? During the main phase of a magnetic storm the elec- tron fluxes decrease dramatically. However, examining fluxes is not the appropriate way to interpret the data. One needs to examine the phase space densities at con- stant first and second invariant. This requires using mag- netic field models. The present field models are wrong during the storm main phase because of the strong cur- rent systems that are operating and the fact that the magnetosphere is usually compressed. Accurate time- dependent models of the magnetic and electric fields are required to put the physics analyses on solid footing. Single-point satellite measurements and ground-based measurements cannot provide the data required to gen- erate such models. New observation and analytical tech- . · , niques are required. One technique that has been suggested is to use energetic particle and field observations from small con- stellations of satellites to derive, self-consistently, the field models. This would be done by adjusting a field model to agree with the multipoint field observations. Then the particle trajectories would be calculated in phase space and the models further adjusted to bring the satel I ite-to-satel I ite mappi ngs of the particl es i nto agree- ment with the observations. Thus the multipoint mag- netic field and energetic electron and ion observations can, in principle, be used to generate a self-consistent and dynamic field model on particle-drift-period time scales instead of satellite-orbital-period time scales. How Does the Electric Field (Convection PatternJ Dynamically Evolve During Storms? Similarly, combining the derived dynamic field model with multipoint electric field and plasma mea- surements may allow inferring an electric field model for the inner magnetosphere. To determine this evolu- tion successfully requires a sufficiently dense and glo- THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS belly distributed set of field and particle observations throughout the inner magnetosphere such as would be provided by an inner magnetosphere constellation-type mission. This mission would require significant devel- opment of data assimilation and modeling techniques beyond what is presently possible and a range of new and develop) ng tech nologies. PLASMASPHERE Achievements Some of the recent ach ievements in understanding the plasmasphere include the first global images by the IMAGE spacecraft and their use to characterize cavities within the plasmasphere. IMAGE has obtained the first global images of the plasmasphere at time scales (10 mint sufficient to track its dynamics. The images are of He+ emissions at 30.4 nm (resonantly scattered sun- light). The IMAGE data have verified the theory of plasma tai Is and revealed a shou Ider-type feature that forms in the dawn sector following sharp northward turnings of the IMP and then corotates with Earth through the dayside. IMAGE EUV images have revealed density cavities that form within the plasmasphere and corotate with Earth; these cavities have been inferred from in situ observations. They have been shown to be the sites for generation of kilometric continuum radiation. EUV and ENA images from IMAGE have shown that the observed ENA emissions from the ring current are diminished in- side the plasmapause and inside the plasma tails. Cor- relative measurements with Polar have shown that these effects are caused by changes in the pitch-angle distri- butions of ring-current ions in regions of higher cold plasma density. Outstanding Questions How Does the Plasmasphere Respond to Changes in Global and Local Electric Fields? The large-scale structure and size of the plasma- sphere depends on the interplay between the global con- vection and corotation electric fields and ionospheric refilling. We know that to the first order the plasma- sphere expands during intervals of low, steady geomag- netic activity (i.e., when there is a low and steady con- vection electric field) and that the plasmasphere shrinks during intervals of high geomagnetic activity. However, statistical studies of plasmaspheric structure repeatably show large variations i n plasmaspheric extent for essen-

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS tially all levels of geomagnetic activity. In addition, sig- nificant structure as a function of local time is observed in response to both stormtime and substorm electric fields. What Role Does the Plasmasphere Play in ULF Wave Generation, Modulation, and Propagation? The plasmapause is thought to act as a natural cav- ity for global ULF wave oscillations and as a sharp boundary for ULF waves propagating into the inner mag- netosphere. Unfortunately, there have been few in situ simultaneous observations of both the ULF waves and the cold thermal plasma of the plasmasphere. In a few cases, such as from CRRES, large-amplitude ULF waves were observed with corresponding density fluctuations, suggestive of the plasmapause boundary moving back and forth across the spacecraft. The modulation of ULF frequency and amplitude has also been observed across the boundary. However, no systematic study has been made of the ULF wave environment near the plasma- pause. The advent of grou nd-based U LF resonance tech- niques that are able to identify the plasmapause location from the ground should address the role of the plasma- pause in modulating and generating ULF waves. What Is the Role of the Terrestrial Plasmasphere in Modulating Ring Current and Radiation Belt Particles? Through Coulomb collisions and wave-particle in- teractions near the plasmapause, ring current and radia- tion belt particles can be lost. In addition, it has been suggested that ULF wave drift resonance may be respon- sible for the energization of relativistic electrons. Again, there have been very few observations of simultaneous U LF, VLF, energetic particle, and thermal plasma popu- lations, so the role of the Plasmasphere in modulating ring current and radiation belt particles is not clearly known. What Is the Mass Composition of the Plasmasphere, and How Does It Change? Measuring the thermal mass composition is experi- mentally very difficult, so little information exists for the mass composition of the plasmasphere. Observations of energetic particle composition and thermal plasma data from instruments on the Dynamics Explorer spacecraft have given inconsistent mixing ratios for helium and hydrogen, though ratios from 0.01 to 0.1 are most com- mon. Essentially there are no statistical studies of the dynamics of the mixing ratios of the heavy ions as a function of geomagnetic activity, so it is generally as- sumed that the mixing ratio is slowly varying and nearly 73 constant. This assumption is needed to interpret the IMAGE EUV observations, which measure resonantly scattered sunlight in the He+ 30.4-nm line. The develop- ment of pal red, grou nd-based magnetometer chai ns and the ability to routinely infer inner magnetospheric mass density should be able to address this fundamental ques- tion. SOLAR Wl N D I NTERACTIONS WITH WEAKLY MAGNETIZED BODIES The solar wind interaction with weakly magnetized bodies exhibits a wide range of individuality and rich- ness in its physical processes. Moreover, one cannot generally separate the aeronomy or sometimes the geology of these bodies from the solar wind interaction. Each member of this class, including Venus, Mars, the Moon, Pluto, asteroids, comets, and interplanetary dust, constitutes a different realization of the parameter space of space-plasma interactions. We are fortunate that a few spacecraft missions have been dedicated to the detailed exploration of the plasma interactions at these bodies. Without the comprehensive earlier views provided by Explorer 35 and the Apollo 15 and 16 subsatellites at the Moon, the Pioneer Venus Orbiter (PVO), and the cometary missions ICE, Suisei and Sakegake, VEGA 1 and 2, and Giotto, our ability to infer what is happening at Mars, Titan, and Pluto from limited new observations and to plan future exploration efforts would be greatly compromised. Nor would we have appreciated, except by conjecture, the compari- sons and contrasts to Earth's solar wind interaction- such as the effects the solar wind interaction may have had on the evolution of the terrestrial planet atmo- spheres. These alternative worlds give us insight into the breadth of plasma interactions that must be present at the still unexplored solar system bodies and in any extra- solar planetary systems harboring terrestrial planets. They also provide insight into the mechanisms of inter- action between the solar wind and the terrestrial mag- netosphere and ionosphere. Some of these mechanisms are universal and operate in magnetospheres of quite different characteristics. Others are specific to particular objects. Hence both the similarities and the dissimi- larities between different magnetospheres can provide deeper insight into the operation of such mechanisms. The basic parameters determining the features of the plasma interactions of weakly magnetized planets are the presence or absence of a substantial atmosphere and ionosphere; the presence or absence of significant remanent magnetization, or magnetic fields induced in

74 the body by the plasma interaction; the nature and prop- erties of the incident magnetized plasma flow; and the size of the body relative to the size of the incident par- ticle gyroradii. Here the panel provides a brief summary of current knowledge, outstanding questions, plans, and recommendations for each of the weakly magnetized hod ies. Venus Achievements Venus exhibits a foreshock that has the general char- acteristics of the foreshock of Earth, scaled for Venus. The bow shock itself is the shape and scale expected for the solar wind conditions at Venus and an obstacle slightly larger in cross section than the solid body since the effective obstacle is the ionosphere. The bow shock has an elliptical cross section, with an average termina- tor radius of about 2.4 planetary radii. The relationship between the bow shock radius and the nature of the obstacle became an issue when it was found that the shock moved inward (by ~0.3 planetary radii at the ter- minator plane) at solar minimum. That amount of change could not be wholly explained by the apparent changes in the Venus ionosphere over the solar cycle. PioneerVenus Orbiter observations showed that the ionosphere's upper boundary ranges from average alti- tudes of ~300 km subsolar to ~800-1,000 km at large solar zenith angles around solar maximum, a height and shape determined by pressure balance between the THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS normal component of the incident solar wind pressure and the thermal pressure of the ionospheric plasma (see Box 2.8~. In the inner magnetosheath, at the boundary with the ionosphere, the initially dynamic solar wind pressure is transformed to magnetic pressure in a layer of enhanced magnetic field and depleted solar wind plasma known as the magnetic barrier. The name ionopause is appropriate for the ionosphere boundary because ions created from the atmosphere at higher alti- tudes are picked up and removed by the solar wind. The ionopause moves up and down with changing solar wind pressure, reaching a minimum altitude of ~225 km, where it extends into the photochemically domi- nated region of the ionosphere. The ionopause thick- ness also varies with altitude, exhibiting a local oxygen ion gyroradius scale of tens of kilometers when the ionopause is high in the collisionless region, but a thicker, more structured and diffuse character at low altitudes, where collisional diffusion and ionospheric convection determine the force balance in the boundary layer. This is in effect an extension of theVenus magneto- sheath into the ionosphere (see the sister report by the Panel on Atmosphere-lonosphere-Magnetosphere Inter- actions), a situation unique to the weakly magnetized planets. The magnetosheath of Venus is well described by magnetized fluid models of flow interaction with a blunt obstacle or conducting sphere. In fact, because the in- teraction system is large compared with solar wind plasma kinetic scales but small compared with the scale of solar wind variations, because the ionospheric ob- stacle is less comprehensible than that of a magneto- The Pioneer Venus Orbiter (PVO) mission is responsible for most of our current knowledge of the Venus-solar wind interaction.This spacecraft operated in orbit for ~14 years (1978-1992), covering both solar maximum and solar minimum conditions at altitudes suitable for observing the foreshock, bow shock, magnetosheath, and magnetotail. The upper atmosphere and ionosphere were regularly sampled with in situ instrumentation only during the early high solar activity phase of the mission owing to PVO's orbit evolution. PVO space physics-related instruments included a magnetometer, solar wind plasma analyzer, thermal ion mass spectrometer, retarding potential analyzer, Langmuir probe, and plasma wave antenna. Local solar EUV fluxes were derived from the Langmuir probe photoemission. The combination of the mission duration and comprehensive payload together with a substantial period of well-supported data analysis,includ- ing a healthy guest investigator program resulted in a revolutionary view of the solar wind interaction with an obstacle that contrasts sharply with the Earth because of its (then confirmed) almost complete lack of a planetary magnetic field of either internal dynamo or remanent nature. PVO demonstrated how an unmagnetized ionosphere was formed and trans- ported and how it interacted with the solar wind. Many unexpected phenomena were discovered such as magnetic ropes in the otherwise magnetic-field-free ionosphere.

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS sphere, and because there are no boundary effects from reconnecting planetary and external magnetic fields, Venus may provide the most ideal example of a mag- netosheath in our solar system. Complications that have been observed include (1) occasionally turbulent mag- netosheath character when the IMP orientation places the quasi-parallel bow shock, with its associated waves and turbulence, near the nose of the bow shock and the stagnation streamline and (2) possible effects of plan- etary ion production in the ionopause vicinity and throughout the upper atmosphere, which extends well i nto the dayside magnetosheath and sol ar wi nd. The subject of cometlike ion production in the solar wind interaction region, reinvigorated by PVO observa- tions of the wake of escaping planetary O+ pickup ions, has proven particularly important for present and future space physics investigations at the weakly magnetized planets. As at comets, the induced magnetotail of Ve- nus, made up of highly draped magnetosheath flux tubes that sink into the wake created by solar wind flow diver- gence around the dayside ionosphere, rotates with the IMP from which it is largely constructed. In addition to its role in induced magnetotail geometry, the so-called mass loading of the solar wind plasma by planetary ions is suspected of having both large-scale effects on the magnetosheath flow and field and small-scale effects such as the production of plasma and MHD waves. What has attracted most interest, however, is the appre- ciation that solar wind erosion of the planetary atmo- sphere has had long-term, evolutionary effects. The mainly atomic oxygen pickup ions at Venus have I arge gyrorad i i (~1 pi anetary red i us) rel ative to Venus, with the consequence that their trajectories inter- sect the exobase (~200 km altitude), where they interact collisionally with the atmospheric gas. This interaction leads to both energy deposition and sputtering. The patchy, weak UV (130.4 and 135.6 nm) aurora observed on the nightside of Venus could be related to planetary pickup ion precipitation, but there are also other candi- date explanations (see the report of the Panel on Atmo- sphere-lonosphere-Magnetosphere Interactions). Model extrapolations of this process into the past indicate that the combination of ion pickup and sputtering-induced escape, together with photochemical escape, has con- tributed to losses of atmospheric constituents like oxy- gen over time. In a striking recent observation that in some ways confirms the historical view thatVenus is in effect a comet, the Ulysses spacecraft detected the Ve- nus pickup ion trail at a distance of over 4 x 107 km (~0.3 AU). These processes at Venus also have direct analogies at Mars. 75 Outstanding Questions The Venus solar wind interaction was so well char- acterized by a single, well-instrumented mission, PVO, thatVenus has not been a high-priority target for space physics measurements for many years. However, PVO raised, and left unanswered, a number of key questions about the Venus solar wind interaction. What Is the Rate of Ion Pickup from Venus? No PVO instrument was designed for pickup ion composition measurements over a sufficiently broad energy range. Pickup ions are originally neutral atoms and molecules that become ionized in the flowing plasma surrounding the planetary obstacle. These ions generally represent a significant loss of atmosphere inte- grated over the age of the solar system. Their energy ranges up to four times that of the solar wind protons times the mass of the ion relative to the proton mass. While some low-energy pickup ions may have been detected by the thermal ion mass spectrometer near the ionopause, and the solar wind plasma analyzer inferred the existence of O+ up to ~8 keV, picked-up O+ energies are expected to range from ~0 to 60 keV. Thus the extent of the pickup ion popu ration at Venus, together with its composition and variations with solar and solar wind conditions, remains undetermined. How Much Sputtering Occurs from the Venus Atmosphere? The sputtering mechanism, whereby neutral atoms are knocked out of the atmosphere by energetic pickup and solar wind ions, has been postulated to exist only through modeling and has not been observed. It may be an important atmospheric loss mechanism. A survey during a range of solar activity conditions with an ener- getic (~100 eV to 100 keV) ion mass spectrometer, complemented by solar wind and IMP measurements, is necessary to fill the gaps in our knowledge of the direct solar wind erosion of the atmosphere of Venus. A low- energy, highly sensitive neutral particle detector or UV spectrometer measurements could resolve the issue of the presence of and importance of the sputtering mecha- nism. Complementary in situ measurements of the iono- sphere below would significantly enhance the depth of interpretation by providing the information needed to model the photochemically produced exosphere that both seeds the pickup ion population and feeds photo- chemical escape. At this writing no Venus missions car- rying such space physics measurements are under de- velopment, although missions are now being planned in Japan and Europe.

76 Achievements Mars Mars exploration in general has suffered from mis- sion mishaps, including the recent losses of Phobos 1 (with the mission of Phobos 2 not being fully realized), Mars 96, Mars Observer, Mars Climate Orbiter, and Mars Polar Lander, although the latter carried no space- physics-related instruments. Most of our current knowl- edge of the Mars solar wind interaction comes from the Soviet Phobos 2 (see Box 2.9) and our own Mars Global Surveyor (MGS). The interpretation of the observations obtained on these missions has been enhanced by observations of the upper atmosphere and ionosphere, by the Mariner 9 and Viking missions, by data from the Soviet Mars mission series, and by Mariner 4 observa- tions of the close-in location of the bow shock and magnetosheath with magnetometers and plasma ana- lyzers. These latter observations from Mariner 4 and Soviet Phobos 2 data were sufficient to establish the weakness of any global magnetic field and magneto- sphere of Mars. Despite its limited period of operation, Phobos 2 provided a detailed and accurate picture of the solar wind interaction with this weakly magnetized planet and its ion loss processes. In addition, several hints at the existence of the crustal magnetic fields of Mars and their effects on the solar wind interaction in the pre- MGS era can be identified in retrospect. The martian obstacle seemed wider than its Venus counterpart, sug- gesting contributions to the obstacle pressure other than those present at Venus. The magnetotail boundary and THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS bow shock also exhibited greater variability in their positions, although with the scale of the Mars subsolar magnetosheath approaching that of a solar wind proton gyroradius, kinetic effects on the solar wind interaction could not be ruled out as the cause. Although MGS is primarily a remote-sensing, surface-mapping mission, the magnetometer and elec- tron detectors it carries provided key solar wind inter- action information that was improved by late changes in the early mission plan. A series of very low periapsis (~110 km), highly elliptical aerobraking orbits during which data were recorded revealed the presence of strong, localized crustal remanent fields strong enough to be significant at the inferred martian obstacle bound- ary (see Box 2.10~. The uneven distribution of the crustal fields, with the strongest features in half the Southern Hemisphere, in the oldest exposed terrain is the likely explanation for much of the observed variability of the tail boundary and bow shock positions. Moreover, the low-altitude, dayside magnetosheath interface with the ionosphere is compl icated by the remanent fields, wh ich produce localized magnetospheric cusp or cleftlike fea- tures, and a hybrid magneto/ionopause that changes with the orientation of the interplanetary magnetic field, as well as with incident solar wind pressure and solar EUV flux. The MGS electron data confirm that the day- time upper ionosphere has a lumpy boundary that reflects the crustal field distribution, reaching higher altitudes over the regions where the field is strong enough to deflect the solar wind above it. Thus, Mars is a lumpy obstacle to the solar wind, and its interaction with the solar wind is probably the most complicated of all of the solar wind interactions sampled to date. Phobos 2 carried several ion spectrometers,an electron analyzer,and a magnetometerthrough several highly elliptical transfer orbits that sampled the subsolar magnetosheath to ~850 km altitude. The ability of the ion spectrometers to distinguish planetary oxygen ions from solar wind protons made it possible to detect a transition in ion composition in the inner martian magnetosheath, which is thought to resemble the Venus situation at solar minimum. There were also a number of inferred "boundaries,"whose probable connection with the later-discovered crustal magnetic fields was not apparent at the time. In its final, nearly circular orbit at ~2.7 Mars radii,data were obtained that led to better understanding of the bow shock positions near the terminator and the induced character of the martian magnetotail. As at Venus, ions of apparent plan- eta ry origin, including O+ and other products from a largely CO2 atmosphere,were observed escaping in the planet's wake. Mars's foreshock also included waves consistent with the presence of a population of picked-up planetary protons.Whereas Mars has an oxygen exosphere or corona analogous to that at Venus, unlike Venus its primary exospheric constituent is hydrogen, detected in Lyman-alpha emission during the early Mariner missions.

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS 77 The Mars Global Surveyor (Figure 2.10.1) was the first spacecraft to enter the martian crustal field magneto- sphere.The Mars-solar wind interaction resembles a cross between the solar wind interaction with the lunar crustal magnetic fields and the Venus-solar wind interaction, which is purely ionospheric. The study of this solar wind interaction is tremendously complicated by the fact that the oncoming solar wind encounters different crustal field configurations as Mars rotates,as well as bythe usual varia- tions due to the changing solar wind and interplanetary magnetic field (with which the crustal fields reconnect). FIGURE 2.10.1 The orientation and magnitude of the measured magnetic field on an GIGS pass through the martian crustal field magnetosphere. Courtesy of M. Acuna, NASA Goddard Space Flight Center. Outstanding Questions How Does the Solar Wind Interact with a Body Having an Atmosphere and an ionosphere in the Presence of a Strong, Patchy Remanent Magnetic Field? Different scenarios for the Mars-solar wind inter- action must exist for each combination of solar wind and interplanetary field conditions and for each longi- tude of noon Mars local time. It is as if a different, com- plex magneto/ionospheric interaction exists for each. Without sophisticated models of the solar wind inter- action, it will be extremely difficult to interpret the data, no matter how comprehensive. The Mars-solar wind interaction thus demands a new mission paradigm where modeling of the observations is not a discretion- ary choice but a necessity for obtaining results. It is also important in Mars studies to be able to use the models, together with the present-day observations, to infer the role the ancient magnetic field of Mars may have had on limiting Venus-like atmosphere escape by ion pickup and sputtering. The ISAS Nozomi mission, on an extended cruise to Mars, where it will arrive in 2004, is expected to carry out an enhanced PVO-like solar wind interaction and aeronomy survey. Nozomi is equ ipped to undertake the needed observational reconaissance with its fu 11 comple- ment of plasmas, fields, and remote sensing instruments. Like PVO, the measurements will cover the upper neu- tral and ionized atmosphere to ~150 to 200 km altitude, together with plasma, plasma wave, and magnetic field measurements throughout its elliptical orbit (up to its ~4 Mars radii apoapsis). While the lower inclination of Nozomi's orbit will limit its in situ perspective to the low-latitude solar wind interaction, the detailed mea- surements will give an unprecedented picture of the subsolar obstacle boundary. NASA's contribution to Nozomi of an ion-neutral mass spectrometer ensures at least some level of involvement by U.S. scientists in the analysis and interpretation of the Nozomi data. ESA's Mars Express mission, launched in June 2003, is expected to arrive at Mars about the same time as Nozomi goes into orbit. The opportunity for collaborative efforts between the two missions has been made part of their science planning. This is especially fortunate be- cause Mars Express includes an ENA detector and ion and electron analyzers to study the solar wind inter- action through a combination of in situ measurements and remote sensing, but does not include supporting magnetic field measurements. On the other hand, Mars Express is i n a h igh i ncl i nation orbit that wi 11 provide in situ ion and electron measurements to complement those from Nozomi's low-latitude sampling, including measurements of heavy pickup ions, which are expected to have asymmetric spatial distributions due to their very large gyroradii compared with the radius of Mars. NASA has contributed the electron analyzer and part of the ion

78 analyzer system for Mars Express through the Discovery mission of opportunity option. Thus U.S. investigators will also be able to participate in the interpretation of Achievements Mars Express sol ar wi nd i Interaction data. Together, Mars Express and Nozomi measurements can bring the state of knowledge of the Mars solar wind interaction to the level of, and in some respects beyond, our current knowledge of the Venus solar wind interac- tion. However, an important caveat is that the Mars so- lar wind interaction is much more complicated, as men- tioned above. Beyond these missions, Mars Odyssey, which arrived in late 2001, includes a radiation envi- ronment monitor to measure the energetic particle and photon fluxes that are potentially hazardous to humans. However, this experiment is not designed to investigate which physical processes determine the radiation envi- ronment at Mars. Except through supporting measure- ments that may come from the MGS, Nozomi, and Mars Express i nstru meets, the Odyssey expert ment wi 11 not allow identification of the conditions (e.g., flares, coro- nal mass ejections, some unforeseen local effect) that give rise to radiation enhancements. Indeed, the ques- tion of what space weather is I i ke arou nd Mars has i m- plications for more than just human exploration. The early Sun may have been more like today's active Sun, with the result that many more energetic particles were present in the solar wind than typically are today. The effects of such fluxes on the martian atmosphere and surface have not been determined. This area thus re- mains open to further enquiry. What Is the Nature of Mars's Patchy Magnetic Field? Opportunities are needed to better map the crustal magnetism of Mars and to send compact but well-con- ceived space environment packages on Mars orbiters in the Mars Surveyor and Mars Scout programs to reveal more about the complexity of the Mars obstacle and Mars space weather. What Is the Particles and Fields Environment of the Surface and LowerAtmosphere, and How Does It Vary with the Solar Cycle? As we learned from PVO atVenus, the solar activity cycle is an important factor in determining the solar wind interaction characteristics by virtue of its effects on both the ionospheric obstacle and the interplanetary conditions. In addition, landed magnetometers, electric field and radiation detectors, and photometers can reveal the extent to which the lower atmosphere and solid body of Mars are engaged in (or respond to) the solar wind interaction. THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS Moon The lunar solar wind interaction was explored to a first level of overall understanding during the Explorer 35 and Apollo 15 and 16 subsatellite missions. Those observations consisted of basic magnetic field and plasma measurements, together with suprathermal elec- tron measurements that could be used to diagnose the near-surface field via magnetic reflection signatures in their pitch-angle distributions. Several landed solar wind experiments were also deployed during the Apol lo years. As expected for a roughly spherical, solid rock body largely devoid of significant atmosphere, the Moon's solar wind interaction signature was generally that of an insulating absorber. This absorption left a wake in the solar wind with an only minimally perturbed interplan- etary magnetic field but few solar wi nd particles i n the immediate vicinity of the Moon. However, strong, local- ized crustal remanent fields, weaker than but akin to those at Mars, were observed to deflect the solar wind away from parts of the lunar surface, especially near the limb of the plasma interaction. The Moon thus repre- sents another distinctive class of obstacle in the spec- trum of solar wind interactions Mars-like in some re- spects, but without a significant atmosphere. Most recently, the Wind spacecraft, which serves as both a solar wind monitor for terrestrial magnetosphere interactions and a magnetotail probe, sampled the lunar wake at a distance of ~6.5 lunar radii with sophisticated ion and electron analyzers, an electric field instrument, and a magnetometer. This combination of measurements provided detailed diagnostics of the ion and electron distribution functions, highlighting their differences and kinetic aspects of their interaction with the Moon. In particular, the observations showed that the solar wind ion wake is approximately coincident with the optical shadow cylinder, while the faster, smaller gyroradius electrons are absorbed on the interplanetary flux tube penetrati ng the Moon. I n the ion wake, charge- separation electric fields acting to refill the wake add to the production of anisotropic particle distributions that produce a variety of plasma waves. The Wind lunar observations clearly demonstrated the value of sophisti- cated plasma and field instrumentation in diagnosing the physics of solar wind interactions. The Lunar Prospector mission provided close-in (~100 km altitude) detections of the solar wind and suprathermal electron behavior in the vicinity of the Moon's remanent magnetic field concentrations, to-

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS "ether with measurements of the field itself. These data revealed that some of the perturbations in the lunar solar wind interaction, inferred earlier from limb features, re- sembled miniature magnetospheres abutting the lunar surface. Outstanding Questions How Does the Solar Wind Interact with the Small-scale Field on the Lunar Surface? We understand I ittle about the solar wind interac- tion with the small-scale fields of the Moon. It is impor- tant that we do achieve this understanding, for a similar interaction occurs at Mars and at asteroids. It is surpris- ing that these regions act as mini-magnetospheres given that the scale size of the magnetic features is similar to the solar wind ion gyroradius. The scale of the solar wind proton gyroradius relative to the size of some of these features would seem to prohibit anything resem- bling the fluidlike interactions at their global magneto- spheric counterparts. The variety of these local inter- actions is daunting, as each small magnetosphere is determined by the strength and orientation of the crustal feature, the feature's position with respect to the sub- solar point, and the conditions in the solar wind, a situ- ation not unlike that at Mars. In all the lunar plasma interaction cases, there is the additional variety provided by the different incident flow types in Earth's foreshock, the magnetosheath, and occasionally in the disturbed magnetosphere. What Is the Nature of the Lunar Wake? The Moon is within striking distance of Earth-orbit- ing spacecraft, as well as spacecraft heading off on inter- planetary trajectories. Despite this accessibility we do not understand well the rather straightforward creation of the wake downstream of an atmosphere! ess, u n mag- netized body. Given the absence of plans for further solar wind interaction exploration of the Moon, it is important to consider what opportunistic use could be made of other spacecraft with instrumentation for par- ticles and fields. The Wind spacecraft has sufficient fuel to change its orbit periodical Iy, for example, and possesses a proven instrument complement for probing the lunar wake. If it is simply a matter of timing transfers of the Wind orbit, possible lunar flybys deserve serious consideration in the planning. Similarly, the twin STEREO spacecraft will use the Moon for gravity assists to achieve their heliocentric orbits. 79 Pluto Outstanding Question What Is the Nature of the Solar Wind Interaction with Pluto? We have no observations that pertain to the solar wind interaction with Pluto. Pluto is thought to be com- posed of an ice and rock mix more characteristic of comets than of asteroids or solid planetary bodies. Our experiences from exploring the solar wind interactions with these other obstacles, together with the assump- tions about Pluto's size and extended atmosphere de- rived from remote sensing, suggest that the Pluto in- teraction may vary from cometlike to Venus-like to Moon-like, depending on a number of factors. A comet- like interaction is indicated if and when the hypoth- esized outflow of ~1 o27 particles per second of atmo- spheric neutrals occurs, perhaps near Pluto's perihelion at ~30 AU. On the other hand, if the ionosphere of Pluto is at these times sufficient to balance the incident par- ticle pressure, some Venus-like deflection of the solar wind may occur, albeit with at least ion kinetic effects coming into play given the Moon-like size of Pluto and the relatively large solar wind proton gyroradius at this heliocentric distance. There is also expected to be a significant pressure contribution in the local solar wind from picked-up interstellar ions, including H+, He+, O+, N+, and C+. These would supplement Pluto's own picked-up ion population from its extended atmosphere of molecular nitrogen, methane, and carbon monoxide (at least) in creating a diverse and highly anisotropic incident flow. As a result, Pluto's environment may be filled with plasma waves of various kinds, similar to the extended region of waves observed around the comets we have visited. Pluto must also be diminished by the constant cometlike erosion of its volatile content, even though its atmosphere will continue to be replenished for some time by sublimation, outgassing, and perhaps particle sputtering. Pluto's orbital period of ~248 Earth years and its significantly elliptical orbit, with perihelion at ~30 AU and aphelion at ~40 AU, are expected to combine with the ~11-year solar cycle variations to modify the atmo- sphere, ionosphere, and thus solar wind interaction on long time scales. On shorter time scales, the interaction must be affected by the ~100 percent variations in solar wind density observed in the outer heliosphere by the Voyager and Pioneer spacecraft. In addition, the pres- ence of some remanent magnetization of Pluto cannot be ruled out. If such magnetization were to exist, even at the ~10 nT surface level, it would also divert incident

80 flows and add to the sensitivity of the Pluto obstacle the large excursions in solar wind dynamic pressure associ- ated with the above density variations. In all cases, the presence of the satellite Charon, located at ~17 Pluto radii, is likely to contribute further to any asymmetries in the solar wind interaction, depending on its own nature as an obstacle as well as its effects on Pluto's atmo- spheric distribution. NASA recently solicited and received proposals for a Discovery-style Pluto Express mission to carry out the first exploration. In spite of stringent constraints on mis- sion cost and science payload, it is notable that particles and fields instruments were considered in several mis- sion concepts. Because the likelihood of returning to Pluto in the decades to follow is so remote, the inclusion of space particles and fields measurements is essential to the completion of NASA's reconnaissance of solar system-sol ar wi nd i Interactions and thei r associated pro- cesses such as atmosphere escape. PSee note, p. 1 23.1 Asteroids Achievements The solar wind interaction with asteroids represents a limiting case in the solar system. There is typically no significant atmosphere, with particle and/or photon sput- Achievements tering and outgassing only weak sources. These solid rocky bodies, with their small (from tens to hundreds of ki I ometers) seal es, provide al most no i mped i ment to the solar wind flow. The existence of lunarlike wake fea- tures is unlikely given the object scale relative to the solar wind proton gyroradius. The generally nonspheri- cal shapes and rotations of these bodies further prevent any orderly wake features from forming on a quasi- permanent basis. A weak solar wind deflection by asteroids has been suggested by observations from Galileo's close flybys of Gaspra (radius ~14 km, closest approach ~1,600 km) and Ida (radius ~30 km, closest approach ~2,400 km) and Deep Space 1 's close flyby of Brai I le. Perturbations in the magnetic field seen at the times of these flybys might be the signature of "whistler wings," formed by an obstacle with scale intermediate between the electron and proton gyroradius. However, the NEAR spacecraft magnetometer detected no apparent Eros-related pertur- bations at that body. Similar searches for signatures of solar wind interaction effects near the asteroid-like martian satellites Phobos and Deimos with the MGS magnetometer and electron detectors produced no clear evidence. THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS Outstanding Question How Do Small Bodies Create Disturbances in the Solar Wind? Three small asteroids Gaspra, Braille, and possi- bly Ida have been reported to cause magnetic pertur- bations in the solar wind flow. Our understanding of how these bodies create these disturbances, as in the case of the lunar magnetized regions, is poor. Further, it has been suggested that the asteroid Oljato produced a statistically significant set of signatures in the interplan- etary field. In addition, the natural variability of the in- terplanetary magnetic field makes identifications of weak signatures from these obstacles difficult without significant supporting plasma measurements in specifi- cal Iy targeted regions such as the near-object wake. The largest asteroids, I i ke Ceres and Vesta, represent the best chance of observing a signature with limited space physics instrument capability. Ceres, nearly spher- ical and with a radius of ~480 km, and Vesta, with a radius of ~260 km, have recently been selected as tar- gets of the Dawn Discovery mission, which will carry a magnetometer. Kuiper Belt Objects The Kuiper Belt objects (KBOs) are probably Pluto- like or cometary nucleus-like bodies composed of ice, rock, and dust. Like Pluto, KBOs have not been observed insitu. Sixty KBOs have been detected since 1992,2 though many others are expected to be present in a disk that extends through and beyond the outer solar system at ~30 to 50 AU. The Centaur family of KBOs includes those that cross the orbits of the outermost planets on their highly eccentric paths. It is frequently speculated that KBOs are leftover planetesimals from the early solar system, and that they are the reservoir for short-period comets. In this scenario, the KBOs become comets when their orbits are perturbed by occasional strong gravita- tional interactions with the outer planets. Other comets may come from the Oort cloud, which is more isotropi- cally distributed around the solar system at much larger d istances. KBO sizes, derived from their brightness and as- sumptions of a comet-like surface character, are in the 2 N RC. ~ 998. Exp/oring the Trans-Neptunian So/ar System. N ationa~ Academy Press, Washington, D.C.

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS asteroid size range, tens to hundreds of kilometers. An even smaller (~1 to 10 km) population is inferred from the short-period comets. Surveys suggest that there are up to a billion KBOs. Telescopic and spectroscopic ob- servations of some of the Centaurs (e.g., Chiron) indi- cate there are short-lived comet-like outbursts of gases and dust from some KBOs due to processes such as sublimation. Outstanding Question How Do Kuiper Belt Objects Interact with the Solar Wind? The plasma interactions with these bodies are ex- pected to be as complex and changeable as the interac- tion at Pluto, depending, of course, on the plasma and field environment which in the case of KBOs includes the outer solar system and possibly, on occasion, the region of the hel iospheric termination shock as wel I as on the occurrence of an outburst that produces a tran- sient atmosphere. The interaction range is at its most extreme when the object is also on a highly eccentric orbit I i ke the Centau rs. While it is unlikely that a mission solely to the Kuiper Belt will be flown in the foreseeable future, plan- ning for a flyby of a KBO has generally been a compo- nent of any Pluto-Charon mission plan. Solar wind inter- action studies generally require a relatively close flyby, but ionization and pickup scale lengths are larger in the dim sunlight and weak magnetic field of the Kuiper Belt region. Comets Achievements In the mid-1980s, the Soviet VEGA 1 and 2, ISAS Sakigake and Suisei, and ESA Giotto spacecraft were sent toward Comet Halley, destined to pass upstream and observe the solar wind interaction in detail, while imaging the cometary nucleus at perihelion. In the meantime, the ISEE-3 solar wind monitor of Earth's mag- netospheric interaction was renamed the International Cometary Explorer (ICE) and diverted to encounter Comet Giacobini-Zinner near peribel ion, on a trajectory that passed through the comet's wake. The data ob- tained by these missions provided a revolutionary view of comets and cometary processes at other bodies. The comet-solar wind interaction appeared more or less as Alfven had predicted decades before. The signa- ture of a highly draped interplanetary magnetic field, produced by the near-stagnation of the solar wind by 81 heavy cometary ion production in the outflowing ex- tended cometary atmosphere, was clearly a major fea- ture. This "mass loading" of the solar wind has counter- parts in the inner magnetosheaths and magnetotails of Venus and Mars, and probably in those of Pluto. The picked-up planetary ions, moreover, extended over mil- lions of kilometers. They were accompanied by a vari- ety of wave popu I ations generated by the h igh Iy an iso- tropic distribution functions of the picked-up ions. Indeed, the cometary environment provided an excep- tional laboratory for pickup-ion-generated wave studies and wave particle interactions. Because of the large ex- tent of the mass-loading regions, it was possible to ob- serve the pickup ion distributions evolve from ringlike to shell-like as the particles were scattered by the waves in their path. An extended cloud of high-energy ions was also observed, evidently resulting from stochastic accel- eration of a small number of the particles caught up in the cometary wave field. G iotto came to with i n ~600 km upstream of Hal fey's nucleus, observing the pileup of the IMP, including the convected rotations of the magnetic field that Giotto had encountered earlier in the undisturbed solar wind. This close approach allowed sampling of the detailed features of the inner boundaries of a strong comet. A magnetic cavity was found surrounding the nucleus at ~4,500 km. Because the measurements were fairly com- prehensive, it was possible to determine that this feature was not a Venus-like ionopause, but rather a boundary where the incident pressure was balanced by ion-neu- tral friction in the outflowing atmosphere. Apparent stag- nation outside this boundary reinforced the early image of a comet as a source in a surrounding flow. Interest- ingly, the ion density profile remained smooth through- out this interface, being photochemical Iy control led. Because these observations occurred when Halley's at- mosphere production rate was strong, at ~103° particles per second, these observations are of course specific to a strongly outgassing comet at peribelion. The contrast with Giacobini-Zinner proved extremely valuable in this regard, as it had a peribelion gas production rate ~100 times less. ICE flew through the wake of Giacobini-Zinner (G-Z) at a distance from the nucleus of ~7,800 km. Although both the G-Z and Halley encounters were characterized by large regions of cometary ions and their associated waves, the G-Z wake flyby was distinguished by the detection of a narrow plasma sheet (thickness ~2,000 km) of cold dense plasma in the center of the draped magnetotail field. ICE, VEGA 1 and 2, and Sakigake also had plasma wave instruments that allowed detection of

82 ELF (~10 to 2000 Hz) and VLF (~103 to 1 o6 Hz) waves in addition to the ion cyclotron, mirror mode, and other low-frequency MHD waves observed with the magne- tometers. These relate to different types of plasma insta- bilities that involve electron kinetics. Both whistler waves and probable lower hybrid waves were observed, often modulated by the larger-scale, lower-frequency waves from the pickup ion instabilities. The richness of the wave phenomena observed at the comets is sti l l not fully appreciated. Outstanding Question How Does the Solar Wind Interaction with a Comet Vary with Heliocentric Distance? In spite of the windfal I of information obtained from the Hal ley and Giacobini-Zinner encounters described above, comets present a moving target in more than a dynamical sense. Thei r solar wi nd i Interactions must rap- idly evolve as they approach the Sun, changing in the extremes from asteroid-like in the most distant reaches of their orbits, to Pluto-like, to something that can de- flect the solar wind at an outflow boundary like Halley's, even after heavily mass-loading the oncoming plasma and field with its own atmospheric ions. Moreover, each comet has its own intrinsic character, including volatile content, shape and size, and structural detail. A recent THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS discovery of soft x rays from comets suggested the possi- bility of using x rays as a new remote-sensing diagnostic of cometary interactions with the solar wind (see Box 2.1 1~. The x rays are generated when charge exchange occurs between heavy solar wind ions and the cometary gases, leaving excited solar wind ions that radiate x-ray lines from a region of the coma sunward of the nucleus. However, the contributions of both the varying solar wind fluxes and the varying cometary gas production in producing the x rays makes it difficult to use this signa- ture for detailed studies. The future observations of the comet solar wind in- teraction largely rest with ESA's Rosetta mission. Rosetta is capable of a fu I I complement of sol ar wi nd i Interaction measurements, including particles over a broad range of energy and composition, magnetic fields, and waves. Its mission is to follow the development of the comet Wirtanen from ~3.5 AU unti I peribel ion, providing the first opportunity to observe the evolution of cometary features with hel iocentric distance. Dust Achievements The physics of solar wind and other plasma interac- tions with dust has numerous important consequences C.M. Lisse and co-workers found unexpected soft x-ray and extreme ultraviolet emissions (photons <2 keV) coming from Comet Hyakutake's sunward coma during its 1996 appa- rition.These emissions from a region ~100,000 km in extent, observed using ES~s ROSAT satellite, could not be easily ex- plained by standard emission mechanisms. T.E. Cravens pro- posed that charge exchange between heavy (e.g., O. C, N) solar wind ions and the cometary neutrals could be responsible (Figure 2.11.11.The process leaves the participating highly charged solar wind ions in excited states that emit the ener- getic photons as they decay.This mechanism explains the ob- served morphology, intensity, spectrum, and temporal vari- ability of the cometary x rays. FIGURE 2.11.1 Isophotes of x-ray emission observed by ROSAT, superimposed on a visible light image of Comet Hyakutake, 1996 B2. Courtesy of T.E. Cravens, University of Kansas.

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS but is often less appreciated than it should be because of its exceptionally cross-disciplinary nature. Dusty plas- mas are found near the surfaces of the Moon and aster- oids; in cometary comas and tails; in the ionospheres of Earth and the planets; in planetary magnetospheres, where satellite volcanism and the sputtering of satellites and rings provide sources; and in interplanetary space, where both cometary and interstellar dust contribute to the zodiacal dust cloud. The plasma-dust interaction can take many forms and thus has many different conse- quences. Dust that becomes charged because of the effort of a grain exposed to plasma and solar UV to balance the currents to and from its surface (by incident particle fluxes countered by photoemission, for example) can act like a highly charged, superheavy ion compo- nent, mass loading the plasma, contributing significant additional gravitational and radiation pressure to the force balance in the plasma and producing a new class of possible waves and instabilities. This charged dust affects the appearance of comet tails, creates structures like spokes in planetary rings and levitated dust clouds on the Moon, and is accelerated in the outer planet magnetospheres by the co-rotation electric field to pro- duce an escaping dusty planetary wind. It has been sug- gested that charging played a key role in grain adhesion and growth in the earliest stages of the planetary accre- tion process. It has also been suggested that interplan- etary dust absorbs and re-emits solar wind ions as a singly charged population, adding to the heliospheric pickup ion sources. Together with the pickup ions from the ionization of interstellar gas, these ions are expected to provide the seed population for the acceleration of anomalous cosmic rays in the outer heliosphere. Outstanding Question How Do Dusty Plasmas Behave? Dust detectors are now an accepted part of many planetary and interplanetary missions. The Galileo, Ulysses, Cassini, Rosetta, and Stardust missions all in- clude them. Furthermore, charged dust is thought to be responsible for phenomena seen in Saturn's rings. De- spite the importance of dusty plasmas and the great in- ternational interest in them, very little research has taken place in the United States. Given the broad applications and potential of a cross-disciplinary initiative on dust in planetary systems to provide important insights in space physics, it would be worth mounting such an initiative in the form of targeted investments by research and analysis programs across space physics, planetary sci- 83 once, and origins of planetary systems disciplines, in- cluding the development of dust detection technologies, laboratory research on dust-plasma interactions, remote sensing of dust in the solar system, the interaction of the solar wind with dust stream plasmas, and the role of grain size. It is important to learn the composition, dis- tribution, variability, and charge state of dust in different solar system contexts and the roles that dusty plasmas and plasma-dust interactions play in solar system phys- ~cs. Epilogue on Solar Wind Interactions with Weakly Magnetized Bodies From the above descriptions of past, current, and planned efforts, it is clear that the United States has ceded to Europe and Japan its previous leadership role in space missions dedicated to the solar wind interac- tions with weakly magnetized bodies. This was appar- ently a NASA management and advisory committee de- cision. Because there are scientifically important observations to be made (as in the examples described above) that the U.S. scientific community can contrib- ute to and gain from, it is recommended that NASA seek and support collaborative ventures that both exploit the opportunities offered by our international partners in these endeavors and keep U.S. scientists and engineers involved in the solar wind interactions research at Ve- nus, Mars, and some comets. It is also recommended that compact particles and fields packages be devel- oped for inclusion on the smaller planetary missions now in vogue. While these missions will never be able to match a PVO in terms of gaining comprehensive knowledge, they open the door to important new in- sights and discoveries such as those obtained on MGS with the magnetometer and electron reflectometer. We must aggressively take advantage of the current prefer- ence for limited missions or forever lose our place in this fruitful area of space exploration. OUTER PLANETS The detection of strong radio signals from Jupiter quickly led to the realization that Jupiter had a strong intrinsic magnetic field and an extensive magnetosphere and radiation belt. Similar signals were not seen from Saturn, Uranus, and Neptune. In the 1 970s the explora- tion of the outer solar system began with the launches of Pioneer 10, Pioneer 11, and then Voyagers 1 and 2. These spacecraft confirmed the existence of a strong Jovian magnetic field and intense radiation belt and then

84 revealed what ground observations could not, that Sat- urn, Uranus, and Neptune also had intrinsic magnetic fields and vast magnetic envelopes. In the following sec- tions, we describe some of the achievements in the study of these magnetospheres and outl i ne the outstand i ng questions. Jupiter Achievements The first pass of Pioneer 10 through the Jovian mag- netosphere, whose noon-midnight cross section is shown in Figure 2.7, revealed a magnetosphere quite u n I i ke that of Earth. The i n nermost part of the magneto- sphere, like Earth's, was dominated by the strong intrin- sic magnetic field, but the radiation levels far exceeded those of Earth, limiting the time that spacecraft could survive in that environment. Further out but inside the orbit of lo, a cold dense plasma (the "cold torus") was . . : 40 ~ 20 a ~ O o 0 ~ -20 53 ~ -40 _ . ~ _ -60 _ I_ Current Sheet- 1 1 1 1 60 40 20 0 -20 -40 -60 Distance Along Jupiter-Sun Line Jovian Radii] FIGURE 2.7 The noon-midnight cross section of the Jovian mag- netosphere illustrating the stretched out fields in the magneto- disk. Courtesy of C.T. Russell, University of California, Los Angeles. THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS found, and outside the orbit of lo a hot torus was found. Both tori were derived from the interaction of lo's volca- nic gases with the magnetosphere, but it was unclear how either torus was produced and why they differed so greatly in temperature. The torus plasma is accelerated to corotate with Jupiter by currents linking the torus to the ionosphere. This removes angular momentum and energy from the planet and transfers it to the plasma. The centrifugal force of the plasma pushing outward against the field then drives a giant circulation system that eventually takes the mass-laden flux tubes to the tail, where their ions can be released from the field lines by reconnection and the emptied flux tube can return to the neighborhood of lo. Jupiter provides our strongest example of a centrifugally driven magnetosphere and is the closest prototype we have to an astrophysical system that sheds angular momentum into its surroundings. Despite the addition of vast amounts of plasma into the magnetosphere by lo, the magnetic field of Jupiter is found to dominate the plasma forces until almost the orbit of Callisto near 25 RJ, where the magnetic field suddenly becomes stretched out in the equatorial plane dominated by the centrifugal force of the rapidly rotat- ing cold plasma. This magnetodisk extends out to about 50 RJ in the dayside and down into the tail at night. A magnetic cush ion region separates the magnetod isk from the magnetopause. Beyond the magnetopause, like at Earth, a magnetosheath, bounded on the sunward side by a strong bow shock, carries the shocked solar wind around the magnetospheric obstacle. A single pass of Ulysses in 1992 explored the higher latitudes of the magnetosphere and confirmed the picture built upon the Pioneer and Voyager low-latitude passes. In 1995 a communication-crippled Galileo space- craft proved the value of an orbiter for magnetospheric studies. Repeated passes of the spacecraft past lo re- vealed the extent of the mass-loading region. The vari- ous processes contributing to the dynamics of the mag- netosphere were revealed, such as temporal variations in the volcanism on lo, reconnection in the magnetotail, and solar wind pressure variations. Optical measure- ments with the Hubble Space Telescope and other 1 -AU observations and imaging with Galileo, as well as with the Cassini spacecraft as it flew by on the way to Saturn, show a dynamic auroral zone over both polar caps. Later the Galileo orbiter revealed the very interesting magnetosphere-within-a-magnetosphere of Jupiter's moon Ganymede (Box 2.121. Plasma circulation, driven by both internal and external processes, appears to be capable of causi ng the low-altitude acceleration pro- cesses leading to auroral forms; these are seen both in

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS 85 Ganymede, one of the Galilean moons of Jupiter, is remarkable in many ways. An ice-covered body, it is the largest known moon in the solar system. In 1996, the Galileo Orbiter discovered that Ganymede has a permanent internal mag- netic field large enough to form a magnetosphere (Figure 2.12.11.The magnetosphere shields the Moon from direct interaction with the flowing plasma of the Jovian magnetosphere within which it is embedded.The discovery of an intrinsic magnetic field in a small planetary body that was thought to have solidified fully over its geological history is a surprise that has led to substantial rethinking of our ideas of planetary evolution.The small magnetosphere has been well charac- terized in multiple passes, at altitudes between 200 and 3,000 km. Analogies with and differences from the terrestrial magnetosphere have enabled us to test and extend our theories of magnetospheric processes. FIGURE 2.12.1 Cuts through the prime meridian of Ganymede representing the magnetic field and Galileo trajectories in the vicinity of the moon at the times of the first four close passes with different orientations of the magnetospheric field of Jupiter at the position of Ganymede.The heavy curves separate magnetic field lines that link to Ganymede at one or two ends from field lines that link only to Jupiter.These curves provide good estimates of the location of the magnetopause boundary. Courtesy of M.G. Kivelson, University of California, Los Angeles.

86 the closed magnetospheric regions and in the open tail regions. Outstanding Questions What Is the Relative Importance of the Solar Wind Interaction and lo-Powered Plasma Transport in Generating Magnetospheric Disturbances? The observations in the Jovian magnetosphere have seldom been obtained with simultaneous solar wind data outside the magnetosphere, and most of the mea- surements have been limited to the equatorial plane. This has left unresolved the important question of the relative importance of the solar wind interaction and lo- powered plasma transport. What Is Responsible for the Observed Structure and Dynamics in the Plasma and Particles Deep in the Jovian Magnetosphere? The formation of the cold torus is still poorly under- stood. The relative importance of scattering losses and radial transport from the hot torus and middle magneto- sphere is not known. In addition, the dynamics of the radiation belts and their long-term and short-term varia- tions are still mysterious. How Does the Magnetic Flux Return to the Inner Magnetosphere from the Magnetotail? The lo interaction loads magnetic flux tubes with heavy ions, and these tubes gradual Iy spiral outward, eventually reaching the near-tail region. There, recon- nection appears to produce magnetic islands of dense plasma that move downtail, leaving closed magnetic field lines connected to Jupiter and nearly devoid of plasma. It is not understood how these depleted flux tubes return to the region of lo against the outward flow of loaded tubes. Saturn Achievements Saturn has been visited by three spacecraft, Pioneer 11 and Voyagers 1 and 2. At first glance these spacecraft reveal a magnetosphere much like that of Earth. While the dipole magnetic field of the planet is not tilted much with respect to the rotation axis, the rotation axis itself has a substantial inclination to its orbital plane, much as has Earth's dipole axis. The magnetic field at the surface of Saturn is similar to but slightly less than the terrestrial THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS surface field. The size of the Saturn magnetosphere is huge, a third as large as that of Jupiter. The presence of a well-developed planetary ring system is quite different from the Jupiter magnetosphere but the rings absorb the radiation belt particles in the inner magnetosphere so that the flux of radiation belt particles is not like that of Jupiter but more like the terrestrial flux. Also like Earth, there are strong radio waves with kilometer-long wave- lengths that appear to be controlled by the solar wind. So, too, the plasma wave population greatly resembles that of Earth's magnetosphere. An important distinction between the magneto- spheres of Satu rn and Earth is the presence i n Satu m's magnetosphere of many rings and moons. While the innermost rings may mainly act to absorb the energetic particles diffusing inward to Saturn, the E ring and the moons can supply mass to the magnetosphere, albeit at a lower rate than in the Jovian magnetosphere. Never- theless, this mass addition, accelerated to corotation, may also drive the radial circulation in the magneto- sphere much as lo's plasma source can drive the Jovian radial circulation. This circulation may also contribute to the generation of the aurora, as plasma circulation does at Earth and Jupiter. Titan is a very special moon with a very dense atmosphere. Wh i le I i ke lo it pro- vides much mass to the saturnian magnetosphere, it does not play the same role in the energetics of the magneto- sphere as lo does at Jupiter because it is much further out in a region of very weak magnetic field. Outstanding Question How Is the Saturnian Magnetosphere Powered? While Pioneer 1 1 and the two Voyagers gave us a good overview of the magnetosphere, they did not pro- vide the quantitative data needed to determine how pro- cesses work. We do not know the basic plasma circula- tion system at Saturn, the strength of the ring and moon mass sources for the magnetosphere, or the relative strengths of solar wind driving and mass loading driving of the circulation pattern. We know nothing about the substorm process at Saturn, a process found to play a critical role in both the terrestrial and Jovian magneto- spheres. Does mass loading or solar-wind-driven con- vection lead to saturnian aurora? How important are dust-plasma interactions in the rings and in the mag- netosphere? There is much left to resolve in the Saturn magnetosphere, but the wel l-instrumented Cassini or- biter on its 4-year-plus mission at Saturn should be able to resolve many of these outstanding issues.

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS Uranus and Neptune Achievements Voyager 2 flew by Uranus in 1986 and by Neptune in 1989, finding large magnetospheres supported by magnetic fields with high harmonic content. The dipole field was tilted at a large angle to the rotation axis for both planets. The unusual nature of these magneto- spheres makes them ideal laboratories for understand- ing magnetospheric processes by comparing their be- havior with that of the terrestrial magnetosphere. Overall, theVoyager 2 passes provided only a rudimen- tary picture of the sources of plasma, the presence of auroras, and the nature of plasma waves at these two planets. Outstanding Question How Do the Magnetospheres of Uranus and Neptune Compare with Those of Earth,Jupiter, and Saturn? Our knowledge of the behavior of these two mag- netospheres is at a very primitive stage. We know noth- ing about their dynamics, the relative importance of the solar wind interaction versus internal sources of plasma in these dynamics, or how the large tilt of the intrinsic magnetic field affects the dynamics of the planetary magnetosphere. How does the tilt of the dipole field affect the aurora? Do the moons and rings of these plan- ets affect their magnetospheres? These very different magnetospheres could be very instrumental in teaching us about the terrestrial magnetosphere by contrasting the behavior of the various magnetospheric processes. Although no missions are in the planning stages for these two planets, there is much interest in exploring them further. The needed measurements could be made with a simple multiparameter particles and fields package, as proposed for other Discovery-class missions, such as those for Mercury and Pluto. 2.3 PROCESSES INTRODUCTION The anatomical investigation of the terrestrial mag- netosphere is complete. We have probed all the im- portant regions of the solar wind-magnetosphere in- teractions and we know their properties. We do not 87 understand, however, how these regions work and inter- act. What are the processes that govern their behavior? In Section 2.2 the panel reviewed the outstanding ques- tions surrounding these processes according to region within the terrestrial magnetosphere and at different bod- ies in the solar system and generated a perhaps daunting list. If instead we consider similar processes in different settings, we find much commonality. In general these processes can be grouped under three distinct themes: (1) the creation and annihilation of magnetic fields; (2) magnetospheres as shields and accelerators; and (3) magnetospheres as complex, coupled systems. By exami n i ng how the processes i n each broad categoriza- tion interact and contribute to the overall system and by examining the processes acting under different plasma conditions at Earth and at the planets, we obtain a robust understanding of the process and of the system at a quantitative and predictive level. In this section the panel outlines how these three themes can unify and focus the recommended program. It emphasizes Earth as our most accessible magnetosphere, but certain phe- nomena, such as the rotationally driven magnetosphere, must be studied in a planetary setting. Moreover, all planetary data are important because the different con- ditions under which a particular process takes place invariably lead to a deeper understanding of the process as it occu rs on Earth. THE CREATION AND ANNIHILATION OF MAGNETIC FIELDS The solar wind interaction with a planetary mag- netosphere is an efficient generator of magnetic fields. This can be seen by imagining the situation without the solar wind. If the Sun suddenly went dark, stopped pro- ducing the light that warms Earth and ionizes its upper atmosphere and stopped producing the solar wind that confines Earth's magnetic field in the cavity we call the magnetosphere, Earth's field would become dipolar, the radiation belt particles would disappear, and the space around Earth would become a vacuum. All the external current systems would also disappear those set up by the solar wind momentum flux that arrives at 1 AU and the electromagnetic energy that Earth captures. A major objective of space physics is to understand the creation of these magnetic fields and their causative currents. We understand some aspects of the currents well, but many not at all. When the interplanetary field is northward and there is no reconnection with the dayside closed field lines, we know where the magnetopause current will flow and how strong it will be. But when the inter-

88 planetary magnetic field turns southward the current system moves and it changes its strength. We can pre- dict neither yet from first principles even though we have some empirical models. If the interplanetary magnetic field is strong and re- mains southward for many hours, then the plasma deep inside the Earth's magnetosphere becomes energized and a new current system arises called the ring current. This current reduces the field on the surface of Earth and expands the magnetosphere. The two main theories on the origin of the ring current are diametrically opposite. Some maintain that the ring current arises when the magnetosphere is subjected to strong time variations in external conditions, while others maintain that steady conditions are needed for deep injection of particles into the magnetosphere. The largest and most powerful magnetosphere in the solar system, that of Jupiter, also has a ring current. This one is associated with the acceleration of ions pro- duced from the volcanic gases of lo. While no one doubts the origin of these ions, there is an analogous problem at Jupiter about the origin of magnetospheric activity. Which processes are energized by the solar wind, as happens at Earth, and which processes are energized by the rotation of the planet? The resolution of these questions has minor consequences for Earth but major consequences for astrophysical systems. Jupiter and Earth also are major proving grounds for models of the reconnection process. The physics of the reconnection process centers around the x point, or neu- tral point, as illustrated in Figure 2.1. Here the problem is the annihilation of magnetic fields and not so much their creation. Magnetic fields can store energy. Earth's magnetotail and, even more so, the Jovian magnetotail are giant storehouses of magnetic energy. Energy can be extracted from the solar wind and the planet and stored in these regions for later release, a release that can be quite rapid. But what triggers this release? Is it internal Iy triggered or externally triggered? What controls the time scales? Are microscale processes critical to reconnec- tion, or is reconnection governed principally by large- scale conditions and processes? The strong, global dis- tu rbances, known as geomagnetic storms, and the weaker localized events, called substorms, are produced by different solar wind conditions. How do these condi- tions alter the internal workings of the magnetosphere to create these phenomena? Why is there not more of a continuum of disturbance strengths? Reconnection takes place at both the dayside magnetopause and in the tail, but we generally treat the reconnection at the magneto- pause as driven and in the tail current sheet as triggered. THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS Why do the two regions appear to behave so differently, or is our understanding at fault? In short, we have much to learn about both the creation and the annihilation of magnetic fields i n planetary magnetospheres. MAGNETOSPHERES AS SHIELDS AND ACCELERATORS One of the fascinating lessons from magnetospheric research is that magnetized bodies (intrinsic or induced) have a JeLyll-and-Hyde behavior regarding high-energy charged particles. On the one hand, the magnetic field forms a shield around the planet that, through magnetic forces, excludes cosmic rays and other energetic par- ticles from much of the surface and prevents the solar wind from scouring the upper reaches of the atmo- sphere. On the other hand, the dipolelike magnetic field forms a trap for charged particles and enables their ac- celeration to very high energies, through a number of intriguing and cosmically important physical processes. This dual personality shield and accelerator raises a number of compelling questions, questions for which answers are now on the horizon. As described in the previous section, Earth's mag- netosphere is a bubble in space that largely deflects and excludes the supersonic solar wind flow impinging on it. The processes by which this exclusion occurs are themselves fascinating subjects of study, and our obser- vations and theoretical investigations to date have raised important questions: How leaky is this shield? Can the solar wind momentum and energy fluxes be efficiently transferred into the system even if solar wind matter is largely excluded? How does the efficiency of the shield depend on the properties of the impinging solar wind? Answering these important questions requires both a detailed understanding of the processes by which the shield is breached and a global understanding of where and under what conditions the transfer into the mag- netosphere takes place. Similarly, Earth's magnetic shield prevents many cosmic rays and highly energized solar particles from entering much of near-Earth space. Yet such particles do have access to the polar regions of Earth and to the outer layers of the magnetospheric bubble, posing questions of practical significance for humans and technology: How deeply can such particles penetrate into our atmo- sphere? How low in latitude can they reach? What solar wind conditions allow them easier access? How can we anticipate the levels, spatial distribution, and duration of energetic particle enhancements? These and similar questions underlie accelerating scientific efforts to un- derstand the physics that produces space weather. Our

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS growing vulnerability to space weather disturbances makes it increasingly urgent to answer such questions, allowing us to predict and mitigate the effects of poten- tial Iy damaging space radiation. How Earth's magnetic shield works, and how wel I it works, are significant and challenging problems. An equally interesting problem is whatwould happen if this magnetic shield did not exist. Indeed, it may not have existed during episodic reversals of the geomagnetic field over Earth's history. What happened to Earth and its atmosphere during those episodes? How much of our atmosphere could have been lost to solar wind scour- ing? What processes would have come into play? What levels of energetic particle fluxes would have had ac- cess to Earth's surface? What consequences would these effects have had for the viability and evolution of life? The answers to these profound questions will lie in a better understanding of how the solar wind interacts with the atmosphere and its ionized upper reaches un- der conditions of weak or absent intrinsic magnetic fields. To help us to achieve this understanding, nature provides an array of solar system bodies with widely differing intrinsic fields and atmospheres from unmag- netized, airless bodies that are exposed to the direct impact of the solar wind, to comets and planets that "capture" the solar wind magnetic field to form their own induced shield. Already, the brief glimpses we have obtained of these exotic systems show us the value of comparative magnetospheric studies. Extending our sci- entific scrutiny to other such bodies wi 11 greatly aid us in answering the questions about the past and future of Earth's shield. The other side of the magnetospheric JeLyll-and- Hyde personality is the clear propensity for magneti- cally governed systems to store charged particles from various sources and accelerate some of them to very high energies. The cosmic problem of particle trapping and energization is typified by Earth's radiation belts. We have been aware of this region of energetic trapped particles since the flight of Explorer 1 in 1958, and much early observational work delineated its extent, its basic properties, and some hints about how it formed and how it varied with the solar cycle. More recent satellite observations have revealed a completely unanticipated degree of temporal variability to the radiation belts, showing them to be a fundamentally dynamic feature exhibiting formation and decay over a wide range of time scales, including the sudden formation of an entire new belt of radiation within a few hours. These more recent observations have left us with a host of compelling questions about the formation and 89 decay of trapped energetic particle populations: We know that particles can be accelerated suddenly during episodic events as well as more slowly over time through a process of diffusion, but we do not know the relative importance of these processes. Moreover, we know some of the aspects of diffusive acceleration, but we do not know just where and under what conditions nonre- versible processes deposit energy into the trapped popu- lation. Nor do we know which solar wind properties drive the diffusive transport or how it is driven or what determines the time scale. Furthermore, there remain fundamental questions about the source population, about its possible preconditioning, and about how ra- diation belt particles are lost through precipitation into the atmosphere, loss to interplanetary space, wave-par- ticle interactions,or particle-particle interactions such as charge exchange. MAGNETOSPHERES AS COMPLEX COUPLED SYSTEMS A major obstacle in understanding the behavior of Earth's magnetosphere and other magnetospheres as well and an important challenge for years to come is that magnetospheres do not exist in isolation and that they show an internal complexity comparable to that of a human body. They are formed by the interaction with the solar wind and are hence strongly coupled to their surroundings. Perhaps even more important, however, are the internal couplings between the different parts of magnetospheres and the couplings between various pro- cesses that operate on vastly different scales. As in a human body, electrical currents and mass flows govern the transport of information and energy. In contrast to the human body, however, these flows are not confined to channels like blood vessels or nerves. Although currents and plasma flows in magnetospheres are often concentrated in thin layers or tubes also, these layers or tubes form dynamically within the three-di- mensional plasma and field distributions; they can change their locations and they can form newly or dis- appear. Owing to this complex behavior, we still under- stand only poorly the response of the magnetosphere to the equivalent of a local artery blockage that is, the local disruption of the electric current flow across the geomagnetic tail. In some circumstances it may just cre- ate its own local bypass; in others it may cause a major collapse, a magnetospheric substorm. There are indica- tions that a combination of the large-scale structure and the local conditions determines the magnetospheric re- sponse to such clogging, but the mechanism is not well u nderstood.

go Geomagnetic storms and substorms are the prime examples of the complex coupled behavior of magneto- spheres, involving al I of its parts and processes on al I scales. During past decades, we learned a lot about the morphological changes associated with substorms, but problems remain unsolved. We know that the magnetic field orientation in the solar wind plays a crucial role: A sustained southward interplanetary magnetic field en- ables magnetic reconnection at the magnetopause and energy transfer to the magnetosphere and, particularly, to the magnetic tail. The release of this energy con- stitutes the main part of the substorm. It involves the severance of the magnetotail plasma, in theform of a plasmoid ejected down the tail, and the collapse of the inner tail, with changes of the coupled current system that also includes the ionosphere. Induced electric fields in the collapsing inner tail cause particle energization. The coupled current systems also involve currents and electric fields that are aligned with the magnetic field and cause bright auroral features. However, we still do not know how a substorm is triggered. There are strong indications that in perhaps half of the cases a northward shift of the IMF might act as a trigger. But that conclusion actually increases the complexity. Why does a northward shift act as a trigger in some cases but apparently not in others? The ejection of a plasmoid requires magnetic reconnection to oper- ate in the tail. But what are the processes leading to reconnection? It is clear that these processes must be collisionless plasma processes involving gradients on very small scales. The formation of parallel electric fields associated with auroral forms also requires plasma pro- cesses on very small scales. Thus small-scale processes appear strongly coupled with the large-scale dynamics of the magnetosphere. There have been a number of suggestions for such processes but no process has been uniquely identified. The connections between the tail and the inner parts of the magnetosphere and the ionosphere raise major questions about the coupled system. What is the role of substorms in the transport of plasma and magnetic flux from the tail to the inner magnetosphere? Magneto- spheric substorms may occur in isolation, but they are also an important part of big storms. What is their rela- tionship? What is the relationship between tail dynam- ics and auroral features? The concept of space weather intrinsically concerns the global coupled system, involv- ing the impact of coronal mass ejections, magneto- spheric storms and substorms, particle energization, flows, and currents and their closure through various parts of the magnetosphere/ionosphere system. Global THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS MHD simulations of this coupled system have been sur- prisingly successful in modeling certain aspects of this complex interaction but unsuccessfu I in others. The magnetospheric system is a high-Reynolds- number system. Flows in such a system are typically highly turbulent. What is the role of this turbulence in magnetospheric transport? How does it possibly obstruct some forms of transport or enable others? What deter- mines the scale sizes of flows? We have also learned that the magnetospheric plasma ultimately comes from two sources, the solar wind and the ionosphere. How- ever, it is not well understood how these sources con- tribute under various circumstances, and how the plasma composition influences the plasma behavior on small and large scales. A resolution of these problems obviously requires a coordinated investigation of the coupled system, involv- ing both local and large-scale views, in close combina- tion with theoretical models that address both small and large scales and their coupling. Significant insight also comes from the study of nonterrestrial magnetospheres. Learning whether and how analogous processes, such as storms or substorms or auroras, operate under differ- ent conditions will provide valuable clues to promote their physical understanding. 2.4 CURRENT PROGRAM INTRODUCTION To accomplish the varied and ambitious scientific objectives described in Section 2.3, a balanced program of theory, modeling, observations, and data analysis is needed. To answer questions regarding the structure and physics of various regions of the solar wind-magneto- sphere system, in situ measurements of a number of im- portant physical parameters must be made; this means that NASA, which is primarily responsible for the na- tion's civilian space platforms, is the dominant agency in the current program. However, it is a hallmark of the space science disciplines that other agencies also con- tribute key elements to the overall program. Examples include ground-based magnetometer and radar mea- surements, digital all-sky images, and photometric data, which are crucial to identifying the magnetospheric re- sponse to various solar wind stimuli, and magnetic field and particle measurements obtained by instruments on operational DOD, NOAA, and DOE satellites, which provide long-baseline and multipoint monitoring of magnetospheric activity. Moreover, a number of differ-

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS ent agencies have ongoing programs to support the sci- entific analysis of relevant data, as well as related theory and numerical modeling. In general, the activities of the various agencies have not been formally coordinated, although the recent focus on space weather has introduced an increased measure of communication, coordination, and col laboration among the agencies. Rather, it is the scientific commu- nity (which itself is a blend of participants from universi- ties, NASA centers, government laboratories, and indus- try) that provides the glue to connect the independent programs of the various agencies. The research commu- nity provides advisory support to the different agencies, facilitating informal interagency communication. More importantly, the research community draws on all the available resources, combining them as needed, to carry out the scientific process. In recent years this ability to combine observations from disparate sources has been greatly facilitated by rapidly expanding use of the Internet. Data distribution centers, such as those at NASA/GSFC and NOAA/Space Environment Center, as well as the more limited databases at other institutions, have made it possible for individual researchers to access and combine data from many different sources. This development is discussed more extensively in Chapter 4, "Report of the Panel on Theory, Computa- tion, and Data Exploration." PROGRAMS Tables 2.1 to 2.4 list current programs that address problems related to solar wind-magnetosphere interac- tions. For each program the tables give the responsible agency, the status, a brief description, and a summary of the science objectives. Table 2.1 lists the non-NASA programs. These programs are mainly funded by four agencies: NSF, USAF, NOAA, and DOE. Table 2.2 lists NASA's nonflight and suborbital programs. Table 2.3 lists NASA's nonplanetary missions relevant to solar wind-magnetosphere interactions. Table 2.4 1 ists NASA's missions relevant to planetary magnetospheres, both in- trinsic and induced. The funding levels for these various programs are not given. In general, the flight programs represent the largest dol lar commitments, but a large fraction of the funding for flight programs goes to space- craft contractors, launch services, and operations. An examination of Tables 2.1 to 2.4 also reveals important aspects of the current program of research in solar wind-magnetosphere interactions: 1. As noted above, the program is multiagency and, indeed, international in scope. NASA maintains a very 91 active engagement with its partner space agencies in Europe, Japan, and Russia, producing opportunities for non-U.S. contributions to American space missions, as well as U.S. participation in programs devised and led by the other agencies. Such collaborations are extremely valuable and scientifically productive, and for some planet-solar wind interaction goals they provide our only option. Thus it is a concern that they are subject to political difficulties; in recent years, new State Depart- ment rules regarding the International Traffic in Arms Regulations have made international collaborations sig- nificantly more difficult. 2. The current endeavor is a broad-based mix of smal I and large programs, ground-based and space- based, observations, and theory. It addresses a wide range of phenomena and physics involved in solar wind-magnetosphere interactions. And it builds on pre- vious programs, using earlier results to refine the ques- tions to be addressed and the techn iques that shou Id be employed. 3. Much of the current program of solar wind-mag- netosphere research depends on the opportunistic use of resources that were originally dedicated to other ob- jectives (note the "Science Objectives" column), repre- senting a strong leveraging of other investments. This includes measurements obtained by non-NASA agen- cies (e.g., DOE), as well as observations from NASA missions targeted at other science objectives (e.g., ACE, which is a mission to study the mass composition of cosmic rays but which has become the mainstay of solar wind monitoring). It also includes planetary missions to a very limited extent; such opportunities for making comparative magnetospheric studies are of particularly high scientific value. The scientific advances that have emerged from such opportunistic programs demonstrate that, although there is strong pressure within NASA for scientifical Iy focused missions, appl ication of creativity and modest additional investments can often reap major dividends in ways that were not part of the original objectives. 4. Many of the operating flight programs are well past their prime. That such missions are still returning very valuable and in some cases, unique observa- tions demonstrates the cost-effectiveness of extended missions. Moreover, the overlap of these earlier pro- grams with more recent missions greatly enhances the scientific return of both, by providing additional per- spectives and complementary information about vari- ous phenomena. 5. There exists a conscious effort to exploit existing data sets, both at NASA and at some of the other agen-

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98 cies. It is by data interpretation and related theory and modeling that we strategically build on current knowl- edge, so it is crucial that such efforts be adequately supported. This topic was considered in the NRC report Supporting Research and Data Analysis in NASA's Sci- ence Programs: Engines for Innovation and Synthesis (Space Studies Board, National Research Counci 1, 1 998) and is addressed at greater length in "Report of the Panel on Theory, Computation, and Data Exploration," Chap- ter 4 of th is vol ume. CRITICAL NEEDS Tables 2.1 through 2.4 above describe a robust pro- gram that is undertaking significant studies on a broad array of topics. The success of this program is due to many factors. Some of these factors need repeating so that the future program can continue the productivity of the present program and that past lessons need not be relearned. Other recommendations concern changes that shou Id be made. Support for the Analysis of Existing Data Such support would provide for the documentation and archiving of the data and for making the data and results widely available and easily usable. The attention to this topic varies across disciplines. In the planetary arena the Planetary Data System (PDS) has been funded to provide d isci pi i ne-wide standards and d isci pi i ne-wide oversight in archiving and data access. In the terrestrial arena the responsibility has devolved to the project level because there is no space physics or Sun-Earth Connec- tion data system. Missions have finite lifetimes centered on the period over which the spacecraft operates. When missions end, their financial support soon ends as well. This puts their data sets at risk. The National Space Sci- ence Data Center is the ultimate archive for PDS and SEC data sets, but it does not provide the level of docu- mentation or of archival and scientific expertise for SEC data that the PDS provides for solar system exploration data. The experience of the PDS indicates that a distrib- uted, common ity-based arch ivi ng organ ization is the most effective means of providing long-term access to data. Interplay Among Space- and Ground-Based Observations and Data Analysis, Modeling, and Theory In a program such as we have at present new ob- servations can be incorporated into the existing para- THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS digms rapidly and progress is rapid. There are support- ing data from space for ground-based observations, and supporting data from the ground are often available for space-based studies. Models have been developed to test current theories and to allow global pictures to be extrapolated from more localized data. This successful interplay provides an important lesson for the future. Progress in science requires a holistic approach whereby the entire system is examined. Seldom will one new observation in isolation be sufficient to permit the achievement of a breakthrough in this field. Communication Between Programs and Agencies The present strong communication has been due in part to the involvement of a number of the key players and i n part to coord i nation of the agencies, both domes- tic and international, the latter coordinated by the Inter- Agency Consultative Group. This communication helps avoid dupl ication of effort and services, develops synergies, and leverages opportunities. The panel strongly recommends keeping the communication lines between future projects in this field as open as they currently are. Flexibility in Research Programs So That an Unexpected Research Result Can Be Incorporated into the Effort Historically, scientific research has often benefited from serendipitous events. Not all research results can be or should be targeted. When new knowledge is gained, we need to find efficient pathways by which to transfer it to other fields and in turn to gain relevant knowledge from other disciplines. Better Mechanisms for Taking Advantage of Missions of Opportunity Currently NASA makes these opportunities avail- able very seldom compared with the frequency at which the opportunities arise, and the selection process is long and cumbersome. NASA should issue mission-of-oppor- tunity Research Announcements more frequently. Mechanism for Facilitating International Collaborations The present technology transfer regulations serve to stifle such collaboration. They also affect purely domes- tic programs.

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS Education and Public Outreach for the Sun-Earth Connection Program There are many excellent individual efforts under way, but these could be improved with increased com- munication, leveraging, and synergy. 2.5 FUTURE PROJECTS INTRODUCTION Even after the ongoing missions detailed in Section 2.4 are completed, the research community will need a series of synergistic, interagency missions, projects, and initiatives during the next decade if it is to resolve the outstanding problems outl ined under this report's three major themes: magnetospheres as complex, coupled systems; magnetospheres as shields and accelerators; and the creation and annihilation of magnetic fields. These future projects represent the next logical steps toward scientific closure, building on knowledge gained from previous and ongoing projects. The collective program will comprise a core of basic research, complemented by focused, intermediate-size, principal investigator (Pl)-driven missions, rounded out by large, community-wide facilities, missions, and pro- grams. These larger projects will typically require sub- stantial financial investment. In this section, the panel outlines particularly those future projects that demand community consensus and prioritization. The panel recognizes that a healthy science program will always require smaller, Pl-driven initiatives as well. Because these smaller initiatives will be competed for through the peer-review process, they require neither discussion nor prioritization here. The NSF and NASA have separate planning pro- cesses for prioritizing major initiatives. Indeed, these planning and prioritization exercises have benefited and continue to benefit from broad community input. There- fore, to the greatest extent possible, the panel takes the results of those exercises as the starting point for its discussion. It notes also that the nascent National Space Weather Program has provided an impetus for the first deliberate, long-range interagency planning of space physics programs. In that vein, some programs, projects, and missions may span several agencies through coopera- tive funding arrangements. These agencies are the Depart- ment of Commerce/NOAA, the Department of Defense/ Air Force and Navy, and the Department of Energy. 99 In the remainder of this section, the panel outlines the projects that will be needed to answer the outstand- ing questions identified in Section 2.2 in the context of the broader science themes delineated in Section 2.3. The following subsections give thumbnail descriptions of the projects' science objectives along with scenarios for their implementation, vetted at past and ongoing community consensus planning exercises. The panel emphasizes that the planning for future NASA missions has benefited from substantial commu- nity input through the mechanism of internal and exter- nal advisory committees; from Office of Space Science NASA Roadmap committees, which develop missions to achieve NASA's strategic science objectives; and from science architecture teams developing targeted science programs such as NASA's Living With a Star program. The panel has reviewed these inputs from the commu- nity and ideas that featured prominently in earlier plan- ning documents. Based on this review, the panel sets priorities for basic science in both terrestrial and plan- etary magnetosphere-solar wind studies, as well as in the targeted science LWS program. ADDRESSING THE MAJOR THEMES Before itemizing the specific projects required to meet our science goals, the panel reiterates the major themes that motivate the larger science framework. These themes are the fundamental science foundations upon which the individual projects build. After outlin- ing these themes, the panel then describes each of its recommended future projects, shows how they relate to the earlier science questions, and presents its priorities for addressing these questions. Magnetospheres as Complex, Coupled Systems The outstanding questions identified in Section 2.2 require a new type of widely distributed data array that will make it possible to distinguish between spatial and temporal variations. While planetary magnetospheres display as much complexity and coupling as the terres- trial magnetosphere, the need to sample the volume of space under investigation as completely as possible means that complex, coupled processes will have to be studied mainly at the Earth. In order to study the com- plex, coupled system of the solar wind's interaction with Earth, we need both space-based and ground-based facilities. In space, answers to the outstanding questions require fleets of spacecraft arrayed in constellations and missions that can provide global images of magneto-

1 00 spheric plasmas. Similarly, on the ground, either dense arrays of observing platforms (e.g., magnetometers) or next-generation, synoptic remote-sensing tools are needed. Such projects are the next logical step toward understanding how the magnetosphere reacts as a large- scale complex system, building on previous "anatomical" projects that have systematically dissected the system region by region and process by process. The companion report by the Panel on Atmosphere-lonosphere- Magnetosphere Interactions also stresses the importance of distributed ground systems for furthering our understand- ing of the solar wind-magnetosphere-ionosphere system. Magnetospheres as Shields and Accelerators Many of the outstanding questions elucidated in Section 2.2 focus on the ongoing mystery of particle acceleration within planetary magnetospheres. This acceleration comes in many different forms and in many different settings through a host of possible processes, some of which may be driven externally and others of which may be driven by an internal energy conversion process. Jupiter, for example, has the most intense radia- tion belts in the solar system. Closer to home, there exist many compelling and practical consequences of the transient energetic particles trapped within Earth's mag- netosphere. On the other hand, magnetospheres, espe- cially Earth's magnetosphere, also act as a shield by blocking the inflow of external energy and harmful solar energetic particles. To this end, missions are needed that will answer the outstanding questions on key boundary shielding regions as well as on particle energization. Creation and Annihilation of Magnetic Fields Creation and annihilation of magnetic fields is a theme of critical importance to the other two themes. Magnetic reconnection plays a significant role in the coupling between solar wind and magnetospheric plas- mas as well as contributing to the complex dynamics within global magnetospheres. Magnetic reconnection also is an important means by which the magnetospheric shield is compromised and almost certainly plays some role in seeding magnetospheric energetic particles. Even in a magnetosphere such as Jupiter's, where the energi- zation of the plasma and its circulation are derived prin- cipally from the rotation of the planet, reconnection is a critical process in governing the plasma circulation. While the creation and annihilation of magnetic fields occurs in microscopic regions, these microscale pro- cesses control and are controlled by macroscale pro- THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS cesses. Therefore, we require projects that not only probe the fundamental plasma physics of magnetic reconnection (e.g., tight clusters of spacecraft) but also quantify the overall significance of reconnection (e.g., widely distributed constellations of spacecraft and dis- tributed ground systems). It may be some time before we can send clusters of spacecraft to study planetary mag- netospheres, but we should take advantage of multi- mission opportunities such as are arising at Mars at present, and such as the Cassini flyby of Jupiter while Gal i lea was operati ng. PROJECT SUMMARIES Before we can prioritize projects or even indicate what science they could achieve, we must know what these projects do. To this end, the panel summarizes in short descriptive narratives the projects to be prioritized and then presents mission details in Tables 2.5 through 2.8. These projects are in various stages of develop- ment. Those that have already had significant commu- nity planning by targeted science and technology definition teams are shown in italics in the tables; other missions that have had less formal planning, but enough to be endorsed broadly by the space physics commu- n ity, are shown i n roman (regu I ar) type. Anticipated and New NASA Solar Wind- Magnetosphere Projects Pursuit of the themes in Section 2.3 requires multi- point measurements in two different forms. Some of the objectives require closely spaced measurements such as the proposed Magnetosphere Multiscale mission will provide (Figure 2.8~. This mission examines fundamen- tal processes at the gyroscale and will reveal how gyroscale processes couple into macroscale processes such as magnetic flux transfer and magnetic reconfigura- tion. Closely separated spacecraft wi l l also al low us to quantify the nature and significance of turbulence as well as to measure the micro- and macroscale processes responsible for particle acceleration. An analogous mis- sion at low altitudes (Aurora! Cluster) would help under- stand auroral acceleration processes where space-time ambiguities impede scientific closure on outstanding problems. We also need to capture the dynamics of the global system. Just as ground-based arrays can monitor the con- sequences of ionospheric plasma dynamics at the feet of the magnetospheric field, constel rations of satel I ites are necessary to capture the sources of these events in the

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS TABLE 2.5 Anticipated and New NASA Solar Wind-Terrestrial Magnetosphere Basic Science Projects 1 01 Primary Science Objective Measurement Objective Mission/Facilitya Determine microphysics of reconnection, turbulence, and particle acceleration. Determine coupling of microphysics to mesoscale phenomena in Earth's magnetosphere. Understand the global dynamics of the coupled solar wind-magnetosphere system by determining the non linear dynamics, responses, and connections within Earth's structured magnetosphere, and by revealing the physical processes operating on spatial and temporal scales accessible to global circulation models. Determine global response of inner magnetosphere during magnetic storms and Magnetospheric substorms by imaging the proton and electron auroras, the plasmasphere, inner plasma sheet, and ring current with time resolution of 1 minute. Define the size, shape, motion, and occurrence rate of the magnetopause boundary phenomena, which regulate the flow of solar wind energy through the magnetopause. Determine response of the magnetosphere during magnetic storms and Magnetospheric substorms by imaging global Magnetospheric total plasma density within 1 2 RE of Earth. Determine how the global topology of the magnetosphere responds to external forcing and instabilities during magnetic storms and Magnetospheric substorms through remote sensing. Determine the energization processes in the auroral region where the magnetosphere is coupled to the ionosphere. Three orthogonal spatial gradients of Magnetospheric Multiscale (MMS)— three-dimensional plasma and energetic particle distribution functions, vector electric and magnetic fields, and electrostatic and electromagnetic waves from ion cyclotron frequency to electron plasma frequency. Four spacecraft with separations from 10 km to 2 RE and time resolution of 0.75 sec in adjustable orbits that scan the magnetopause and magnetotail. 3D plasma distribution functions, energetic particles, and vector magnetic field from 50 to 100 spacecraft between geostationary orbit and ~25 RE geocentric distance. Science would be enhanced by distributed ground-based system. Image ENAs (1 to 500 keV), EUV at 30.4 nm, and FUV at 121.8 nm and 140-190 nm from two spacecraft in crossed Molniya orbits with apogee at 9 RE while simultaneously making in situ measurements of plasmas and magnetic fields from equatorial orbit with apogee of 10 RE. Vector magnetic field and plasma from a network of ~36 spacecraft skimming both sides of the dayside boundary layer over a wide range of latitudes and local times. prioritized SolarTerrestrial Probe (STP) mission; study team report completed; phase A study begins late 2003; launch in early 2009 Magnetospheric Constellation (MagCon)— next solar wind-magnetosphere STP mission as currently prioritized; study team report can be found at <http:// stp.gsfc.nasa.gov/m issions/mc/mc.htm >; launch in~2012 Stereo Magnetospheric Imager (SMI)—in 1 997 Roadmap Dayside Boundary Constellation (DBC)—in 2000 Roadmap Measure Faraday rotation of electromagnetic Magnetospheric Tomography (MagTom)— waves transmitted from Earth to 16 spacecraft in see <http://sprg.ssl.berkeley.edu/ an elliptical equatorial orbit with apogee of 12 RE. ConstellationClassMissions/ergun.pdf> Image ENAs, EUV at 30.4 nm, and FUV at 121.8 nm and 140-190 nm from either of two spacecraft from fixed position at 50 RE above north and south poles using solar sails or multiple orbiting spacecraft. Particles and fields on a suite of small spacecraft flying in close formation. Geospace System Response Imagers (GSRI) —in 2000 Roadmap; also called Polesitter in 1997 Roadmap Auroral Multiscale (AMS)—in Sun-Earth Connection 2003 Roadmap, which can be found at <http://sec.gsfc.nasa.gov/ sec_roadmap.htm> NOTE: SolarTerrestrial Probes, approximately $400 million, based on estimated cost caps in effect in late 2003. aProjects with significant community planning by targeted science and technology definition teams are shown in italics; those with less formal planning, but enough to be endorsed broadly by the space physics community, are shown in reman (regular) type. magnetosphere. While these spacecraft can be simply i nstru mented, many satel I ites are needed to cover the volume of the magnetosphere in which these events occur. The Magnetospheric Constellation (see Figure 2.9) and the Dayside Boundary Constellation are two such high-priority missions. These missions provide in situ measurements throughout large volumes. Two complementary approaches addressing this same problem of probing the dynamics of the magneto- sphere over a wide volume of space are Magnetospheric imaging and tomography. Both the Stereo Magneto- spheric Imager and the Geospace System Response Im- ager (or Polesitter) respond to this need via remote sens- ing as the next-generation multipoint auroral and neutral

1 02 THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS TABLE 2.6 Anticipated and New NASA Solar Wind-Terrestrial Magnetosphere Targeted Science Projects Primary Science Objective Measurement Objective Mission/Facilitya Determine the processes that energize ring-current Measure three-dimensional particle distributions and plasmas and produce the radiation belt by establishing fields from a small constellation of spacecraft in nested global reconfigurations of fields and distribution of geosynchronous transfer petal orbits at various local particles in the innermost magnetosphere. times. Measure the upstream solar wind and interplanetary magnetic fields and their longitudinal variations associated with coronal mass ejections and interplanetary shocks. Use three small spacecraft carrying plasma, magnetic field, and energetic-particle instruments in Earth synchronous orbit, close to the Earth-Sun line, but well inside 1 AU, powered by solar sails to gradually vary their mutual separation to investigate three-dimensional structures. NOTE: Living With a Star, approximately $400 million, based on estimated cost caps in effect in late 2003. aSee note a,Table 2.5. atom imagers. While the former mission can be done early, technology development is needed for the latter if executed with a solar sail spacecraft. Radio frequency tomography of density structures of the outer magnetosphere is another attractive ap- proach whose technical readiness lies between that of Geospace Probes (GProbes) (also appearing in a larger form as Inner Magnetosphere Constellation)—in 2000 Roadmap Solar Wind Sentinels—in 1997 Roadmap the two imaging missions. A mature mission concept, Magnetospheric Tomography, wou Id i ncl ude up to 1 6 spacecraft (see Figure 2.10~. A highly desirable concep- tual merger of MagCon/DBC and MagTom approaches would allow for both remote sensing and in situ mea- surements. TABLE 2.7 Anticipated and New NASA Solar Wind-Planetary Magnetosphere Projects Primary Science Objective Measurement Objective Mission/Facilitya Determine the consequences of the lack of an ionosphere on the response of a magnetosphere to solar variability. Determine the relative contributions of planetary rotation and of the interaction with the IMP to jovian magnetospheric dynamics; determine how global electric and magnetic fields regulate magnetospheric processes; identify the particles responsible for the jovian aurora and their source region(s). Determine the role that extreme variations in internal magnetic dipole tilt have on the structure and dynamics of a planetary magnetosphere and on its response to solar variability. Determine the role of a strong internal plasma source to a rotating magnetosphere and its impact on magnetospheric dynamics. Determine how the upper atmospheres of terrestrial planets are influenced by the solar wind in the absence of a global magnetic field. Three-dimensional plasma distribution functions, energetic particles, and vector magnetic field on Mercury orbiter. Measure particles and fields in situ in the auroral acceleration region, along L shells, and in the conjugate source regions from an elliptical polar orbit with perijove at 1.1 R. and apojove at 15 to 40 R.. Measure plasmas, energetic particles, vector magnetic and electric fields, and plasma waves and image Neptune's aurora from a moderate-inclination eccentric polar orbit. Low mass particles and fields experiments in a 6 by 70 R., equatorial orbit. Measure neutral species escape rates, isotopic ratios, densities, temperatures, and winds; measure thermal plasma, energetic particles and magnetic and electric fields. Messenger—in 2000 Roadmap, now in development under Discovery program Jupiter Polar Orbiter—in 2000 Roadmap Neptune Orbiter—in 2000 Roadmap lo Electrodynamics (IE)—in 2000 Roadmap as a Frontier Probe Venus Aeronomy Probe (YAP) and Mars Aeronomy Probe (MAP)— both in the 2000 Roadmap NOTE: See note,Table 2.13.The panel points out that in the solar wind-planetary magnetosphere area greater emphasis should be given to space particles and fields experiments as missions of opportunity on planned national and international planetary missions (e.g.,Venus Express, Mercury Orbiter, and the ISAS mission to Venus).The panel also stresses the importance of Pl-class planetary missions studying magnetosphere-solar wind interactions through the Discovery and Explorer programs. Future missions to Pluto, Europa, comets, and asteroids could bring much new understanding of the interactions of these bodies with their plasma environments for a small investment in instrumentation. aSee note a,Table 2.5.

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS TABLE 2.8 Anticipated New NSF Medium-Size Initiatives 1 03 Science Objective Measurement Objective Mission/Facilitya Determine response of high-latitude ionospheric convection, and hence magnetospheric convection, to varying solar wind input. Determine global rapid time variations of magnetospheric currents to obtain density structure and to determine mechanism, time, and location of substorm onset. aSee note a,Table 2.5. A parallel effort in planetary magnetospheres is also critical to a successful program because these com- plementary systems allow greater insight into the fun- damental processes of magnetospheric physics. For STIR—2000~233683 National Aemnaut~cs ancl Apace Actmin~$tral'nn (;od.daFrl Space Ftight Center (ireenlbeli' ~larytand 20771 December 1993 FIGURE 2.8 Cover of the STDT report detailing the science and mission implementation strategies of the Magnetospheric Multi- scale mission. Courtesy of NASA. Measure vector ionospheric convection velocities throughout polar cap using incoherent-scatter radar located within 10° of magnetic pole. Relocatable for flexible use. Measure X,Y, Z components of surface magnetic field at a cadence of 1 Hz in large-scale arrays covering all latitudes. Advanced Modular Incoherent Scatter Radar (AMISR)—formerly the Relocatable Atmospheric Observatory Magneto-Seismology Array (MSA) example, Mercury's magnetosphere (to be probed by Messenger), with its absence of a significant ionosphere and its small size, provides such comparisons. The Jovian magnetosphere provides the contrast of a rotationally dominated magnetosphere and vast size; Jupiter Polar Orbiter will determine the relative contri- butions of planetary rotation and of the interaction with the IMP to Jovian magnetospheric dynamics (see Figure 2.11~. It will also determine how global electric and magnetic fields regulate magnetospheric processes, as wel l as identify the particles responsible for the Jovian aurora and determine their source regions. Neptune provides a magnetosphere ordered by a dipole field that has a very large tilt angle. The Neptune Orbiter mission will determine the role that extreme variations in the internal dipole tilt have on the structure and dynamics of a planetary magnetosphere and on its response to solar variability. Io provides an intense source of mass loading to the Jovian magnetosphere. The lo Electrodynamics mission will determine the role of a strong internal plasma source in a rotating magnetosphere and its impact on magneto- spheric dynamics. Venus and Mars Aeronomy Probes both show how ionospheres can hold off the flow of the solar wind for weakly magnetized bodies with atmospheres. These missions will determine how the upper atmospheres of terrestrial planets are influenced by the solar wind in the absence of a global magnetic field. Finally, in the applied science area the panel sup- ports the plans of the LWS program and those of other agencies to monitor and predict space weather. The Geospace Probes will determine the processes that en- ergize ring-current plasmas and produce the radiation belt by establ ish i ng the global configu ration and evol u- tion of electric and magnetic fields and the distribution of particles of the innermost magnetosphere. Two pos-

1 04 THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS FIGURE 2.9 Artist's conception of the Magnetospheric Constellation capturing the dynamics of the magnetospheric plasma associated with a dipolarization of the near tail and the formation of a plasmoid. Courtesy of NASA. sible configurations of a Radiation Belt Mapper mission are shown in Figure 2.1 2. A key driver of terrestrial space weather is the vari- ability in the solar wind. The Solar Wind Sentinels (SWS) mission will measure the upstream solar wind and inter- planetary magnetic fields and their longitudinal varia- tions both for normal conditions and also for conditions associated with coronal mass ejections and interplan- etary shocks. Anticipated New NSF Initiatives Ground-based facilities provide data on the ultimate fate of the energy and momentum transfer from the solar wind and provide access to regions of the magneto- sphere and parameters that are difficult to acquire from space. Our ability to probe the upper atmosphere and ionosphere from the ground with radar and optical in- struments has expanded greatly over the last several de- cades. The most critical region of the magnetosphere for which such measurements are needed is the auroral and polar regions, at best difficult environments in which to work. A most economical way to obtain these data is with NSF's proposed Advanced Modular Incoherent Scatter Radar (AMISR). It would determine the response of the high-latitude ionosphere to the solar wind input by measuring both ionospheric convection and the heat- ing of the upper atmosphere using incoherent scatter radar and optical instrumentation. It could be deployed at different latitudes and longitudes to address a variety of objectives as problems are solved and new ones are identified. While AMISR represents a large investment at a single site, there is also a need for well-dispersed inex- pensive instruments. The epitome of such a device has been the magnetometer, which has long been used to measure magnetospheric activity. Recent advances in magnetometry (increased precision and lowered costs) and improved analytical techniques have enabled the magnetometer to make measurements of fundamental importance to magnetospheric physics through magnetoseismology. The attempt to use U LF pu Isation data to remotely sense plasmaspheric mass properties has a long history. Since the mass density of the mag- netosphere is critical to the speed at which waves travel through the magnetosphere and to the speed at which the magnetosphere can react to stresses, it is critical to

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS (b) (c) (d) 4 2 us O 11] -2 Oh CD N -4 -64 .. -8 -6 -8 1015 u 0 -5 -10 X GSE (RE) Tomography Data ~ 1 ~ ~ ~ ~ ~ ' E 1014 us 13 ~ 0 20 40 60 80 100 120 Path Number Reconstruction of Plasma Density _0.20 FIGURE 2.10 A simulation of the density maps that the Mag- netospheric Tomography mission could make using radio to- mography techniques in Earth's magnetotail. Courtesy of R.E. Ergun, University of Colorado at Boulder. know its mass distribution. However, most of the plasma is cold and often invisible to space probes. Fortunately we can now probe the equatorial mass density with local techniques via the measurement of resonant fre- quencies, much as solar oscillations allow the Sun to be 1 05 c ~ 0.20 FIGURE 2.11 The Jupiter Polar Orbiter mission will address key -15 questions left unanswered by the Galileo mission. Courtesy of NASA. seismical Iy probed. Also, sudden increases in solar wind pressure can be used, in a manner akin to earthquakes, to probe the density structure of the magnetosphere through the inversion of travel times. These two tech- niques can be used with inexpensive latitudinal chains 0.76 of modern GPS-synchronized magnetometers. The panel recommends that a Magneto-Seismology Array for both types of density sounding be established. Finally, the panel notes the utility of irregularities that reflect radio signals; these irregularities occur often enough to provide a useful tool for studying plasma circulation. SCIENCE TRACEABILITY In this section, the panel illustrates the ways in which projects are directly responsive to our science questions and themes. Tables 2.9 to 2.11 show the flow from the three science themes to specific science ques- tions and then demonstrate how we will reach closure on these questions with a systematic suite of carefully planned projects. The projects identified by name in the tables are prioritized and detailed in the next section. Projects that address primarily a specific science ques- tion are denoted by a P. Those that provide secondary support are denoted by an S. Owing to the broad nature of the themes, several projects are often required to ful- fill the overall theme goal. Accordingly, they may take a decade or so to be completed, principally as a result of funding constraints rather than science drivers. Indeed,

1 06 THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS FIGURE 2.12 Two possible Radiation Belt Mapper constellations: Orbit apogees extend just beyond geosynchronous orbit, and peri- gees are low; orbit planes lie in Earth's equatorial plane in both options. Courtesy of NASA. the panel stresses thatthere will be significant additional value if several projects can be done concurrently rather than serially. PRIORITIZATION: NASA AND NSF Next, the panel prioritizes the projects listed in Tables 2.9 to 2.11. In assigning priorities, it used the following criteria: · Importance of the scientific problem, · Technology readiness, · Adequacy of the theoretical foundation, · Likely impact, and · Appropriate timing of related projects. The panel has sorted the priorities into several cat- egories, appropriate for the different funding agencies and programs under which such projects would fit most natural Iy. These prioritized I ists fol low i n Tables 2.1 2 to 2.14. The panel also lists here several additional projects that are generally supportive of all its science themes. These missions would manifestly contribute to the sci- ence goals of the missions in Tables 2.9 to 2.11, by providing ancillary observations of the solar wind or global auroral imagery. Such missions may be motivated by programmatic issues, but the panel stresses that they would be (and others before them have been) vitally important for achieving the most possible science. The prioritized list of interagency missions follows in Table 2.1 5. PRIORITIZATION OF OTHER AGENCY AND INTERAGENCY INITIATIVES The panel recommends that each agency continue its i nd ivid ual space envi ran ment man itori ng that serves not only the agency itself but also the space science community at large. Such missions include NOAA's GOES, USAF's DMSP, and DOE's LANL geostationary spacecraft programs. These missions form an invaluable resource for the science community by providing ancil- lary data that complement other large programs. The panel endorses and encourages the further collection, archiving, and timely dissemination of these data. Furthermore, the panel recommends conducting the large anticipated missions listed in Table 2.15, all of which may require strong interagency cooperation. While these missions will primarily serve to monitor the space envi ran meet, i n keep i n g with the i r pri nc i pa l ob- jectives, their scientific value is great also. In particular, they may help to provide critically needed solar wind and i nterpl anetary magnetic field man itori ng as wel I as

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS TABLE 2.9 Magnetospheres as Complex, Coupled Systems Project 1 07 Science Question MMS GProbes MagCon SMI DBC SWS MagTom GSRI AMS JPO NO IF VAP MAP RAG MSA What is the global P S P P structure of magnetosheath fields and plasmas' What are the causes P of terrestrial magnetospheric activity, including substorm onset and recovery? What is the connection between terrestrial substorms and magnetic storms? p P S P p p S p p How does the mid P S P S S P and distant tail map to the planet magnetically anc dynamically? What is the role of P P P S the terrestrial plasmasphere in modulating ring current and radiation belt particles? What are the scale P sizes of geoeffective solar wind structures What is the role of P S P P P P P turbulence in Earth's magnetosphere and at its boundaries? Do solar-wind- P P S P P driven phenom~r~ occur in other magnetospheres? What is the relative P S P importance of various sources for populating the terrestrial plasma sheet? NOTE: P denotes projects that address primarily a specific science question; S denotes projects that provide secondary support.

1 08 TABLE 2.10 Magnetospheres as Shields and Accelerators THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS Project Science Question What are the wave S mode transitions in Earth's magnetosheath and how sharp are they? How are shocks modified by multiple components? = MMS GProbes MagCon S What are the origin P S P and destiny of planetary ring current and radiation belt particles? What is the nature of convective energy transport and geomagnetic activity? What is the role of the solar wind in plasma loss at weakly ionized bodies? What are the origin and dynamics of Jupiter's aurora? What is the dynamic evolution of auroral acceleration? = SMI DBC SWS MagTom GSRI AMS p p p JPO NO IF VAP MAP RAG MSA S P P p p P P P P p D NOTE: P denotes projects that address primarily a specific science question; S denotes projects that provide secondary support. global au roral i magi ng both tasks are as of yet on Iy assumed but will be essential to many of the basic and targeted science missions in Tables 2.12 to 2.14. 2.6 TECHNOLOGY INTRODUCTION Many of the solar wind-magnetosphere interaction science studies and science missions, discussed in ear- lier chapters, can be achieved only by applying new technology or developing enabling technologies. The areas in which new technologies are required cut across P P P s all aspects of space science and space engineering. Even many of the near-term science missions in the current program discussed in Section 2.5 require technologies that are not yet space qualified or qualified for flight. There are five broad areas where investment in new technology is required to meet the science and mission needs identified in this study: · Propulsion (e.g., solar sails, solar electric, and nuclear propulsion); · Spacecraft technology (e.g., miniaturization to microsat/nanosat levels, advanced communications, advanced power systems, autonomous operation, mass production, development of a "sciencecraft"~; · Science instrumentation (e.g., miniaturization, advanced detector technology, mass production);

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS TABLE 2.11 Creation and Annihilation of Magnetic Fields 1 09 Project Science Question = MMS GProbes MagCon = SMI DBC SWS MagTom GSRI AMS What is the interplay P P P S S between micro-, mesa-, and macroscale aspects of reconnection? What are the P P P S relative roles of component and antiparallel merging? JPO NO IF VAP MAP RAG MSA What is the global S P S S reconnection rate on the magnetopause and how does it vary? Isreconnection P P P P S patchy or continuous, inherently unstable or quasi-static? NOTE: P denotes projects that address primarily a specific science question; S denotes projects that provide secondary support. · Information architecture (e.g., data synthesis and assimilation, model development, multipoint data visu- alization); and · Ground systems and operations (e.g., advanced operations and data handling systems). TABLE 2.12 Prioritized NASA-led Missions Mission Program Magnetospheric Multiscale (MMS) NASA SolarTerrestrial Probes (STP) All of these are driven by the science requirements and mission objectives spelled out above. In many cases, the science and mission objectives cannot be met using existing technologies. Each of the five areas is discussed in more detail below. For example, the science drivers and mission concept for the Magnetospheric Constella- tion mission require the use of a large number of very smal I spacecraft/sciencecraft. The technology that is re- quired to fulfill this mission alone covers a broad range, from new instrument, satellite, thruster control, and me- chanical assembly technology to new operations capa- bilities and data handling and assimilation technologies. Geospace Probes (GProbes) Living With a Star (LOOS) PROPULSION TECHNOLOGY Magnetospheric Constellation (MagCon) Stereo Magnetospheric Imager (SMI) Dayside Boundary Constellation (DBC) Solar Wind Sentinels (SWS) Magnetospheric Tomography (MagTom) Geospace System Response Imagers (GSRI) Auroral Cluster (AC) NOTE: Approximately $400 million. STP STP STP LWS STP LWS STP Many current and future missions require develop- ment of new propulsion technology, enhancement of existing technology, or the creation of new systems by the marriage of older and newer technologies. For ex- amp~e, new technologies are needed to allow satellites to maintain (non-Keplerian) trajectories that are not pos- sible without continuous thrust being applied. Examples of missions that require such technologies are the Solar Wind Sentinels and the Polesitter missions, which must "thrust" to maintain a proper position relative to the Earth-Sun line and over Earth's polar regions, respec- tively.

1 1 0 TABLE 2.13 Prioritized NASA Planetary Missions THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS TABLE 2.14 Prioritized NSF Facilities Mission Program Mission Program Jupiter Polar Orbiter (J PO) New Frontiers or Discovery Advanced Modular Incoherent Scatter Radar (AMISR) NSF Neptune Orbiter (NO) New Frontiers Magneto-Seismology Array (MSA) NSF lo Electrodynamics (IF) New Frontiers or Discovery Ven us Aeronomy Probe (YAP) Discovery Mars Aeronomy Probe (MAP) Discovery or Mars Scout NOTE: Cost caps in FY 2003 for New Frontiers, Discovery, and Mars Scout were <$700 million, <$350 million, and <$350 million, respectively. Solar sails are a possible technology for doing this. Solar electric propulsion might also achieve the same effect. The limitation of solar electric propulsion is that the satellite must carry fuel for the system, although it would have much less mass than that required by a chemical fuel system. Solar electric propulsion has been spaceflight-qualified by the New Millennium program, but the technology requires further development. Solar sails are an unproved technology that at this point would require significant investment before it is ready to sup- port the missions mentioned. In fact, a privately sup- ported mission may be the first to use a solar sail in spaced Deep space and outer planet missions could use solar sails or a marriage between nuclear power and electric propulsion. Missions that must go very close to or very far from the Sun could take advantage of such propulsion. However, there has recently been signifi- cant reluctance to use nuclear power on satellites and there has been no attempt to develop nuclear electric propulsion. The very deep space missions have no choice but to use nuclear power for the instruments and avionics. Extending nuclear power to the propulsion sys- tem would seem to be a natural evolutionary step. The combination of nuclear power and electric propulsion would enable deep space missions, such as a mission to the hel iopause, to be completed on much shorter time scales than otherwise possible. Some relatively high power nuclear systems have been flown in the past so some technology exists but they are high-mass systems that impose a heavy-lift launch vehicle requirement on missions that use them. The recently announced initia- tive by NASA to develop nuclear power systems for space is clearly a step in the right direction. In addition to the above, it may be possible to com- bine heat engines with electric propulsion for long- duration, near-Sun missions, once the satel I ite arrives 3MarkAlpert, Sailing on sunlight, Scientific American, 285, 1, 2001. near the Sun (having been launched by conventional rockets). However, no such technology is being consid- ered at th is ti me. At this point, solar sail technology is being studied by NASA because it is capable of achieving large veloc- ity changes and requires no fuel. It also meets many mission requirements.4 Many studies of solar sail use have been completed or are in progress. Significant investment is required to develop prototypes and obtain flight experience in time to support science missions. Ultimately, one would like to have a power sail that wou Id provide not on Iy the propu Ision but also the elec- tric power necessary for satellite operation. Another area where propulsion technology needs development is the support of constellation-class mis- sions that require numerous micro/nanosatellites to be placed or scattered throughout a region to obtain the required spatial coverage. Most current concepts require that each satel I ite have a thruster to obtain its fi nal posi- tion, with the bulk of the energy provided by a mother or dispenser ship. While multiple satellites (<10) were dispersed from single launchers in the past (i.e., Iridium and Globalstar), those missions did not have the kind of satellite distribution requirements that are envisioned by a mission such as the Magnetotail Constellation (Mag- Con) mission. At present, the MagCon dispenser ship is a concept only. Significant effort is required to develop dispenser ship technology by the end of the present decade, when MagCon is scheduled to fly. Similar pro- pulsion difficulties exist for other Living With a Star mis- sions, such as the Radiation Belt Mappers, which envi- sion micro/nanosatellites in multiple "petal" orbits. Such orbits are not easily obtained from a single chemical booster. Missions like this may require a dispenser ship, possibly with solar electric propulsion, to distribute the constel ration of satel I ites properly. 4NASA, OSS. 2000. Strategic P/an. NASA, Washington, D.C.

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS TABLE 2.15 Prioritized List of Anticipated Interagency Projects (<$200 million) Science Objective Measurement Objective Mission/Facility Lead Program Monitor the upstream solar wind and interplanetary Remote sensing and in situ plasma and fields Triana—spacecraft NASA magnetic fields as well as continuously observe the instrumentation on spacecraft at forward essentially complete emissions from the daylit hemisphere of Earth for Lagrange point. and awaiting a global change studies. launch to L1 point Measure the far upstream solar wind and interplanetary magnetic fields associated with coronal mass ejections and interplanetary shocks. Monitor global auroral zone emissions to quantify global energy inputs from magnetosphere to ionosphere and to complement constellation-class . . missions. So far, the focus of constellation missions has been to modify existing technologies to come up with a dis- penser ship. However, the large number of satellites in- volved means significant development. Test flights of any such ship would be required before it can be com- mitted to carry a large constellation of satellites. Another technical challenge for constellation-class missions is the ability to track a large number of satel- lites and return their measurements to the ground. Early, limited experience with operating small numbers of spacecraft in a cluster suggests that the operational aspects of a constellation mission may be very different from the operational aspects of traditional science missions. Finally, there is a need for new propulsion technol- ogy to support upper atmosphere and lower ionosphere probing or "dipper" missions. Such missions require relatively high-thrust, low-mass propulsion systems. Cur- rent systems are too low in thrust or too high in mass. They can support only very limited duration missions. SPACECRAFT TECHNOLOGY The major spacecraft technology advances required to meet identified science goals are in micro/nano- satel I ite development, autonomous and robust satel I ite operation, and long life. The need to reduce mass and power extends across all space science missions. Tech- nologies that do so enable or enhance the missions. For example, reduction of satellite mass reduces propulsion requirements, which itself enables some missions. Simi- larly, technology that makes satellites robust and more autonomous reduces the need for operational support. The panel has identified several science missions that require multiple distributed satellites. For example, Measure solar wind plasmas and fields well upstream of the L1 point using solar sail technology to provide earlier warning for approaching geoeffective solar wind structures. Remote sense multiple wavelength auroral emissions relevant to both electron and proton precipitation. Geostorms—several NOAA/NASA concept studies completed Auroral Monitor NOAA/NASA the Geospace Mappers (ionosphere and Radiation Belt Mappers), Dayside Boundary Constel ration, Auroral Cl uster, and Magnetospheric Tomography missions al I require multiple (4 to 16) satellites with a range of capa- bilities, and the MagCon mission requires the largest number (~60 to 1001. All require the development of new multisatellite technology capabilities. The MagCon mission is an example of a science mission that is driving nanosatellite technology devel- opment. It requires multiple simultaneous in situ obser- vations throughout a specific region of space. This drives technology on many fronts. The main challenge is to minimize all satellite subsystems from the mass, size, power, and cost perspectives. Thus, each satellite sub- system function must be examined with an eye to reduc- ing it, combining it with other functions, or eliminating it if possible. In addition, to generate such a fleet of nanosatel I ites on a normal development schedu le re- quires that they be mass produced and tested much as is done today for limited runs of specialty automobiles. For example, the Iridium satellites were produced using assembly-line techniques. ICO, which recently acquired a majority stake in bankrupt rival Globalstar, is an ex- ample of a company having a multisatellite program. However, ICO and G lobalstar produced relatively large and expensive satellite systems. Furthermore, both com- pan ies were wel I capital ized. MagCon requires the development of small and cheap platforms. This challenge may require abandon- ing the usual concept of building a bus to accommodate science instruments developed and fabricated by mul- tiple institutions. Instead, the instruments and satellites would be developed as a system to be produced by one fabricator. This would be a significant departure from

1 1 2 the usual process and would require much new process technology. The phase B portion of such development takes on a greater importance, because there is no room for fixing things downstream in phases C and D of such programs. Considerable investment in new management and design concepts and technologies is required now if large numbers of nanosatel I ites for constel ration mis- sions are to be successfully implemented. Any new man- agement structure must have the instrument Pls as an integral part. The instrument Pls have the ultimate re- sponsibility for sensor design and must work closely with the satellite (or sciencecraft) fabricator to imple- ment the design and support the development of the data hand I i ng and analysis processes. To reduce the mass and power requirements of sat- el I ite subsystems means looking to new components and technologies such as microelectromechanical systems (M EMS) gyros and Sun sensors for attitude determina- tion and microthrusters (thrusters on a chip) for attitude control, where necessary. It also means developing much denser, more capable, and lower-power satel I ite data processing and control subsystems. Technologies are being developed in the commercial world that could be adapted to space systems. Much effort is currently being put into developing low-power, high-density, and h igh-speed appl ication-specific i Integrated ci rcu its (ASICs). These run the gamut of analog ASICs, digital ASICs, and hybrid analog/digital ASICs, and many of them are finding their way into science instruments and satellite systems. Again, investment in these technologies is required to successfully achieve significant reductions in satellite mass and power resource requirements. It should be noted that such technology would enable a range of space science and engineering missions cutting across the whole space science enterprise. Ground station and personnel costs are a significant part of a mission's total cost and extend over long peri- ods. Reduction of cost and simplification of operations requires robust, long-lived satellites, preferably autono- mous to a great degree. Autonomy is primarily a system and software robustness issue. Development of such sys- tems is a long-term endeavor and requires continued investment to advance the technology even as the space- craft and instrument capabilities grow. The autonomous operation of satellites relies on technology that could benefit many missions. For deep-space missions, new communications technology is a pressing need. Inflatable antenna struc- tures may be an enabling technology for such missions insofar as they reduce the ground or NASA Deep Space Network (DSN) system resources required to support the THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS missions. Such antennas could provide the signal gain needed to allow use of very-low-power transmitters for near-real-time measurements in support of the LWS pro- gram. The high-gain signals could allow the LWS pro- gram to use relatively cheap and small dedicated ground receivers instead of DSN to support this component of the program. It must also be recognized that some ex- penditure of technology development funds for the space-based portion of the communication link can alleviate growing pressure (oversubscription) on the ground station. The use of Ka or even optical communi- cations can greatly increase the bandwidth, reducing contact times, but often these systems are dropped be- cause of their initially high development cost. For near-Earth missions, cell-phone-like communi- cation I inks using commercial near-Earth satel I ite con- stellations (such as Iridium or ICO) to retrieve data and support satellite operations would be an enabling tech- nology. Investment is required to solve the problems imposed by limited reception range and Doppler shifts in signal frequencies between the moving platforms. However, commercial communications systems could be a cheaper, more flexible way to support and operate new science missions such as the ionosphere Mappers. Satellite-to-satellite communications technology would be enabling for multisatellite or constellation missions. NASA's Space Technology 5 program has this as one of its technology development goals. Greater use of GPS signals to obtain near-Earth sat- ellite ephemeris on board is an enhancing technology that could reduce the need for tracking and ranging to obtain ephemeris data. Such technology supports mul- tiple near-Earth missions, even beyond the GPS orbital altitude. GPS could also provide the very precise timing signals needed to synchronize simultaneous measure- ments from constellation spacecraft and to perform interferometry. Again, some investment is required before such capabilities become routine and reliable enough to become a common element of science mis- . , . s~on aes~gn. There are several other satellite technologies in de- velopment that should be sustained or accelerated to meet the needs of new science missions. While these technologies are not required, each offers capabilities that can be mission enhancing. They include the devel- opment of I ith i u m-ion batteries and chargi ng systems; active thermal control using electrosensitive surface materials and coatings; miniature star trackers; MEMS gyros and Sun sensors for attitude determination; and new, deployable structures that unfold and become rigid support elements. Each of these is under development in

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS industry and provides a capability that can be used by several science missions. Development progress shou Id be monitored and supported. SCI ENCE I NSTRUMENTATION TECH NOLOGY Science instrumentation struggles with the con- straints imposed by the basic physics of measurements and the need to obtain statistically meaningful data. This difficulty limits the ability to easily miniaturize all sensors. However, there are several technologies in development that could reduce instrument resource requirements. For example, the expanded use of ASICs is dramatically reducing instrument complexity and power while increasing capability. Qualified high- density ASICs lead to more reliable instruments purely because there are fewer parts. Many are in develop- ment and promise to reduce power requirements by orders of magnitude while increasing performance by similar orders of magnitude. These developments need to be identified and supported so the technology is spaceflight-qualified in time to meet science mission needs. Generally, the ASICs, being application specific, are developed for a particular type of processing, such as fast Fourier transforms for wave data and time-of- flight systems for particle sensors. However, such needs cut across al I space science missions, and an ASIC developed for an interplanetary wave instrument can be applied in a wave instrument elsewhere, for example. The advancement of sensor and detector technolo- gies also is a strong driver. The physics of the devices that make the measurement, be it of electromagnetic energy, photons, or particles, becomes the limiting fac- tor for the science. Advances in sensor technology are critical to all science missions. Examples of such sensor development are the 6-doped charge-coupled device (CC D) arrays, both as normal CCDs for imaging systems and as pixelated detectors for miniature low-energy particle detection systems (such as solar wind spec- trometers). Whi le these hold much promise for increas- ing sensor capability and reducing sensor complexity and size, especially for particle detector systems, they require significant investment and effort to make them as accessible as off-the-shelf detector elements like microchannel plate detectors, for example. Uses for MEMS technology in sensors are evolving. For example, MEMS devices could be used to create adaptive sensor geometries that allow an instrument to respond to current conditions, such as highly variable particle or light fluxes, by modifying the effective gather- 1 1 3 ing power of the system. This could reduce the number of sensors required to make a measurement under con- ditions where there is a large dynamic range of possible input. Such capability is important for missions of dis- covery or exploration where the dynamic range of mea- surement parameters that will be encountered is un- known or where best estimates exceed the dynamic range of current sensors. At present, multiple sensors are often flown to cover the full measurement range. Mis- sions to planetary magnetospheres and to the heliopause and missions near the Sun (Solar Probe) are the types of missions where such capability could mean measure- ment success i nstead of fai I u ret Smart processor technology can be enabling for some science measurements. One development that has shown promise is the use of processor technology based on field-programmable gate array (FPGA). This is a fast and adaptive technology wherein program logic is ex- ecuted in FPGA hardware. The adaptability comes from the ability to reprogram the FPGA hardware and obtain processing speeds that are not possible from a software- driven system. Work has been done to develop compil- ers to convert programs into FPGA circuit designs. This technology could have application in spacecraft sys- tems where fast processing and programmability are necessities, such as some deep space missions. At pres- ent, these systems are somewhat bulky and power hun- gry. However the concept is good and the technology is advancing. Continued support is required to maximize the capability of such systems. In many cases, significant investment will be re- quired to continue the evolution of sensor technology. The basic technology exists but must be refined and adapted to the kinds of measurements that need to be made. Time-of-flight sensor technology for plasma and energetic particle composition measurements is one such development. The technique is well known but difficu It to implement. Development of a time-of-fl ight electronics in ASIC form is a goal for the space plasma community, which would bring this powerful technique to missions that do not have sufficient mass and power resources to use it now. As electronic capabilities and material technologies evolve, the science output from missions increases. This is true for optical, magnetic field, and plasma wave measurements also. There must be in place a process to identify the promising technolo- gies and to support their migration into sensor systems. This means there must exist a cross-enterprise sensor technology development program that provides the sup- port to do the migration and requires that the results be made accessible to all instrument builders.

THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS INFORMATION ARCHITECTURE TECHNOLOGY As the density and complexity of information in- crease, science missions are driven by the need to handle and ingest the resultant data. New technologies and concepts are required to make massive amounts of data accessible to science and scientists. The science and engineering community, with support from NASA, needs to continuously evolve this process. There is no clear-cut separation between data processing, data com- pression, data retrieval, data storage, and data distribu- tion and assimilation onboard the spacecraft and like processing on the ground. Different missions will require that different parts of the process be done in different places. For example, a deep space or outer planet mis- sion may send back mostly highly processed data to reduce bandwidth requirements. In future missions there may be a need to assimilate the observational data into models onboard the spacecraft and telemeter the results to the ground. The different technologies used must be migrated to the appropriate platforms. The constel ration-type missions, i n particu far, have a recognized need to assimilate the data into global models. This requires development of software and sys- tems that can take the data inputs in real time indepen- dent of their quality and completeness. The concept is to use adaptive physical models (like adaptive MHD codes) to provide connectivity between the indepen- dent in situ observations and to generate a complete moving picture of the dynamic system being studied. This is not a current capability except for single-point interplanetary data inputs to MHD codes. What the con- stellation science teams envision is to dynamically modify the MHD codes to best represent the totality of the observations. Data from constel ration missions im- pose constraints on the modeling codes. An intensive effort is required to construct codes with appropriate data assimilation technologies. Model development itself is driven by a need for new tech nology, i n some cases a new computational tech nology. I n other cases it is the development of new software technology and numerical techniques. Model development is a cross-enterprise issue, and theoretical and modeling missions should be considered to be as important as experimental missions. TECHNOLOGY FOR GROUND SYSTEMS AND OPERATIONS As noted above, the cost of ground systems and operations can be very large, especially for long-dura- tion missions. Technologies that reduce and simpl ify the systems free resources that can be redi rected i nto the science component of the missions. Just as autonomous satellites can reduce the burden on ground and opera- tions systems, autonomous ground systems can reduce the manpower required for mission support. With sev- eral upcoming missions being multisatellite or constel- lation missions, it is imperative that new operation con- cepts and technology be developed and implemented. Examples of possible technologies can be found in com- mercial communications satellite organizations. Many of these companies run large numbers of satellites with relatively small crews. In fact, these organizations are ahead of NASA in the area of autonomous operations of systems of spacecraft. Their technologies should be stud- ied and emulated where appropriate; otherwise they should be used as a starting point for the development of the operations and ground systems that will be required by MagCon and other mu Itisatel I ite science missions. A separate issue is the retrieval and handling of the science data from modern missions. Again, in the past this has been a manpower-intensive and costly effort throughout the mission. Intensive support is required to find and implement architectures and technologies able to handle the massive increase in data that new missions will generate. New technologies are required to make such massive amounts of data easily accessible by all scientists. The science and engineering community as a whole needs to continuously evolve this process. The different technologies must be migrated to the appropri- ate platforms. RECOMMENDATIONS AND PRIORITIES As noted above, there needs to be a significant fo- cus on developing new instrument, satellite, propulsion, operations, data assimilation, and processing technolo- gies. The top priority in each of these areas is addressed below: · Science measurements. A space-science enter- prise-wide instrument development program is needed that is separate from SR&T budgets. This issue clearly needs to be addressed if quality measurements are to be made on the smaller micro/nanosatellites being envi- sioned for future multisatellite and constellation mis- sions. In addition, new management and team struc- tures must be generated for developing highly integrated, micro/nano, science-craft-type spacecraft. This must be done on a time scale that meets the development needs and implementation of multisatellite missions.

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS · Propulsion. The top priority is to push the devel- opment of solar sail technology to support missions that require non-Keplerian orbits in the interplanetary me- d i u m to make the necessary science measu remeets at appropriate locations. NASA has a good start in this area but needs to move forward with hardware development and spaceflight demonstrations so the technology is flight qualified in this decade. · Satellite technology. The push shou Id be to de- velop fabrication, integration, and testing technology for micro/nanosatel I ites that wi 11 enable constel ration mis- sion science. This requires bringing together new mate- rial and electronics technologies, new management techniques and structures, and new integration and test- ing processes. A way of integrating the development of spacecraft and science instruments is needed. Test-bed- type development programs that take multiple micro/ nanosatellite systems from concept to flight should be considered as one way of developing and testing the new processes. This work needs to be done soon if it is to support missions that are identified in NASA's Strate- gic Plan. · Operations and data handling. The focus should be on implementing evolving technologies that reduce manpower and costs for all missions. In particular, at- tention should be given to the complete data chain, from the operations required to get the science data to the manner in which the data are brought to Earth, as- similated into models, and ultimately presented to sci- entists. This must be started now if the needed capabili- ties are to mature on a time scale that meets planned schedules for complex new interplanetary observatories and mu Itisatel I ite constel ration missions. 2.7 SOLAR WIND- MAGNETOSPHERE INTERACTIONS: POLICY ISSUES INTRODUCTION The need to observe coupled dynamics of the large magnetospheric system in response to solar wind condi- tions has several implications. First, the difficulty of ad- equately specifying the state of the dynamic system means that the number of observing platforms should be as large as possible. This goal would be best achieved by interagency coordination to maximize the opportuni- 1 1 5 ties for instrumentation on non-NASA platforms. Sec- ond, because ground-based observations provide dis- tributed knowledge of convection and boundaries that cannot be achieved in space, ground-based capabilities must not only be sustained but enhanced, and coordina- tion between space- and ground-based observations needs to be exploited worldwide to the fullest extent possible. Third, it is now firmly established that knowl- edge of solar wind conditions is critical to developing further understanding of the magnetosphere-ionosphere system, so that sustained, continuous monitoring of solar wind input is essential. Fourth, global physics-based computer si mu ration codes are an essential component of the research program because they will play a central role in maximizing the information that can be extracted from the observations and unifying disparate data sets within a comprehensive framework. These implications in turn have specific ramifications for the policies that should be pursued to achieve the key science objectives identified above. INTERAGENCY COORDINATION Because the resources required to make the neces- sary observations exceed those available from any one agency and because the societal impact of the science return is relevant to a variety of agencies and interests, efficient coordination between agencies is a preeminent policy concern. In fact, one could argue that the coordi- nated system is much more valuable than the sum of its components. NOAA:Transitioning New Operational Observing Platforms and Models The National Oceanic and Atmospheric Adminis- tration has two roles to play. First, the transitioning of space instrument platforms from basic science research to operational systems needs to be anticipated and implemented in a timely fashion. Since space-based sci- ence platforms are almost exclusively the substance of NASA programs, these transitions will require coordina- tion with NASA. Typically, this is done by taking the key instrumentation and data reduction techniques devel- aped under NASA research programs and implementing them under NOAA. Observations that are or should be planned for transition i ncl ude the fol lowi ng: · Interplanetary magnetic field and solar wind ob- servations analogous to those provided in rea/ time from

1 1 6 the ACE spacecraft at the L1 point. The necessity for continuous IMF/solar wind observations from L1 has been abundantly demonstrated scientifically and opera- tionally. Nearly all predictive models of magnetospheric response depend pri nci pal Iy on IMF/sol ar wi nd i nputs. Since the ACE spacecraft is operating beyond its design life, it would be prudent to implement a new L1 plat- form for this purpose in the very near future (<2 years). Steps should be taken to continue such L1 in situ moni- toring as an operational system. · Solar coronal observations. The dramatic ad- vance in our appreciation and understanding of the causative link between coronal dynamics, coronal mass ejections, and high-speed streams, in particular, and major geomagnetic disturbances made possible by re- sults from the instrumentation on YOHKOH and SOHO has motivated both the Solar Dynamics Observatory of the LWS program and the STEREO mission. There is now little doubt that observations of this class will play a central role in operational space forecasting, and steps should be taken to coordinate operations with NASA in the short term (next 5 years) and to deploy a line of operational monitors in the medium term (5-8 years). The GOES SXI instrument is an important first step. · Auroral imaging. The advances i n u nderstand i ng magnetospheric dynamics, particularly nightside/mag- netotail processes made possible with global auroral imaging, demonstrate the value of these observations for monitoring intrinsic magnetospheric dynamics as well as energy transport to the ionosphere. New results from the IMAGE mission promise to increase our understand- ing of the physical correlates of these observations, thereby improving their operational value. It is already clear that real-time auroral imaging will prove opera- tionally valuable, and plans should be laid for NOAA to provide global auroral imaging on an operational plat- form in the long term (8-10 years). The second role for NOAA concerns the transition- ing of models from science research tools to operational resources. NOAA, NSF, DOD, and NASA al I have theory and modeling efforts in magnetospheric physics. The Space Environment Center of NOAA supports a small effort to transition science models to operational use. Discussions with both agency and scientific community personnel indicate that this transition effort is undersupported. They note that there appear to be many more models available in the community that could be useful to space weather than are being actively con- verted to operational status. THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS A parallel concern exists in the research commu- nity, which has a separate need for standard models. In the next decade, the research community also will re- quire access to realistic global models, including MHD simulations. The Coordinated Community Modeling Center at GSFC5 is providing the first community access to such global models, in part in coordination with the NSF's GEM program. The challenge of providing sophis- ticated models to the community is a significant one and will require an advisory structure to prioritize and ensure maximum efficiency in coordination between agencies and to minimize duplication of effort. The panel recom- mends that these transitioning efforts be supported aggressively to meet the science and applications objec- tives of the space environment community. DOD-DOE: Coordinated Planning for Launch and Flight Opportunities and Access to Relevant Data Sets The Department of Defense and the Department of Energy conduct operational flight and observation pro- grams that are directly relevant to the science objectives of magnetospheric-solar wind interactions. Ensuring ap- propriate use of these resources is an extraordinarily cost-effective means of achieving several of the observa- tional goals described above. Launch opportunities will be avai lable, particularly for sending smal ler payloads, ~300 kg, into geosynchronous transfer and low Earth orbits on DOD vehicles. These are key regions for magnetospheric dynamics, particularly radiation belt dynamics. H istorical Iy, launches of opportunity have proven problematic in practice because of cost con- cerns associated with launch schedules. (The cost growth and resulting cancellation of IMEX were due in part to probl ems of th is sort.) Mechan isms for accom- modating NASA payloads on DOD launch schedules without raising NASA mission costs via prolonged launch delays need to be studied. In addition, agree- ments between NASA and DOD regarding launch op- portunities need to be formalized so that the availability of these opportunities and the mutual commitment to support the programs that use them do not hinge on agency personnel remaining in key positions. Discus- sions with DOD representatives indicate that launch opportunities will continue to be available, but at present arrangements to use these opportunities are made only on an ad hoc or informal basis. 5See <htip://ccmc.gsfc.nasa.gov>.

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS Current, future, and planned DOD and DOE plat- forms obtain or will obtain measurements for opera- tional purposes that are directly relevant to magneto- sphere-solar wind interaction science objectives. These measu remeets i ncl ude data from DMSP and N POESS particle and fields detectors and from auroral imagers, particle data from DOD and GPS satellites, and total electron content from ground-based GPS receivers. These data sets provide valuable augmentation of space- based observations and need to be exploited to the full- est extent possible. Support for preliminary processing and archiving of these data for retrospective scientific analyses is essential if the scientific community is to make meaningful use of these assets. NASA and NSF are urged to coordinate with DOD and DOE to facilitate preliminary processing and archiving activities so that these resources are leveraged to the fullest extent possible. NSF: Central Role for Ground-Space Coordinated Observations Global magnetospheric dynamics are reflected in ground processes. Our understanding of the specific correlation between ground observables and magneto- spheric configuration and dynamics has dramatically increased in recent years, in part through the efforts of NSF's GEM campaign. We now understand the relation- ship between convection distribution and the underly- ing magnetopause reconnection geometry. We also ap- preciate how to identify time variations in reconnection in ground observations. We now know how to relate auroral spectra to the precipitating source population and critical bou ndaries i n the magnetosphere, both i n the magnetotail and on the dayside. Ground-based observations provide distributed measurements of quantities that are difficult or impos- sible to measure with comparable distribution in space. These measurements include radar and ground magne- tometer observations of convection; multispectral and high time and spatial resolution auroral imagery, merid- ian scanning photometers, and ULF pulsations. The ground observations therefore provide a means of moni- toring the enormous system with remarkable efficiency. Historical Iy, NSF assumed the central role in both estab- lishing the ground-based observatories and coordinat- ing these observations with measurements from space. Mai ntai n i ng and expand i ng these grou nd observa- tion assets is critical for two reasons. First, ground obser- vation assets will prove even more valuable as new space measurements are made and as models become able to assimilate these data. Recent advances in identi- 1 1 7 tying ground signatures with specific magnetosphere- solar wind interaction phenomena make the obser- vations even more valuable because they provide quan- titative contextual and distributed information that is key to specifying the system and unavailable any other way. Second, some impediments remain to establishing an unambiguous link between phenomena that can be ob- served from the ground and magnetospheric-solar wind dynamics. These impediments include our incomplete understanding of ionospheric conductivities and the dif- ficulty of specifying the net result of auroral processes that couple the high-altitude magnetosphere to the iono- sphere. Only by comparing extensive ground observa- tions with space-based observations will it be possible to further improve the power of ground-based observa- tions. COORDINATION BETWEEN PROGRAMS AND DIVISIONS WITHIN AGENCIES: NSF AND NASA Because magnetospheric physics is one of a number of priorities in NSF and NASA space science programs, and because responsibility for it is split between NASA and NSF, it is important to recognize and eliminate un- necessary compartmentalization. The panel encourages cooperation and coordination between agencies and between programs within each agency. Several areas in which coordination is desirable are discussed next. Comparative Magnetospheres and Planetary Exploration Comparative magnetospheres remains a vital prov- ing ground for theories of magnetospheric dynamics, because different systems present configurations and conditions not found at Earth. Our understanding of magnetosphere-solar wind physics will be seriously de- ficient unless these extraterrestrial systems are explored in ways that allow us to test our theories of their dynami- cal behavior. Solar system exploration therefore needs to provide avenues for observations of other magneto- spheres. In the past, major solar system missions could ac- commodate planetary geology and atmospheric and magnetospheric science payloads. This has not proven to be true under the Discovery program, whose missions are more highly focused. Instruments whose purpose is to further the understanding of comparative magneto- spheres have less appeal in the Discovery mission envi- ronment than instruments that provide new information on a particular solar system body. The Solar Terrestrial

1 1 8 Probe missions and, to a greater degree, the Living With a Star missions tend to focus on the Sun-Earth Connec- tion rather than on comparative magnetospheres. The panel encourages the Solar System Exploration and Sun- Earth Con nection programs to coord i n ate thei r programs in the upper atmospheres and magnetospheres of the planets and to develop missions that address the out- standing problems in these areas. Ties Across Organizational Boundaries As the science of the solar wind, magnetospheres, and ionospheres matures and a new emphasis on serv- ing the needs of space weather emerges, it is natural that new programmatic structures are being adopted within and between funding agencies. As these new structures are adopted, however, it must be recognized that the physical systems whose study is being overseen are closely linked and do not respect administrative bound- aries. These structures may be reasonable and based on general distinctions between disciplines, but are none- theless artificial constructions. Science disciplines must be allowed the freedom to explore the linkages between these physical systems. As new organizational structures are adopted, the ties between subdisciplines must not be lost, and research that spans administratively differ- ent areas must not be allowed to fall through the cracks. Often, cross-discipl inary research is not given priority by either discipline and therefore languishes. Planetary magnetospheres and solar wind interactions are an ex- ample of disciplines where such a lacuna occurs. A for- mal mechanism to fairly evaluate and support cross- discipl inary research should be adopted. Broadening the categories within NASA's SR&T program to allow mag- netospheric and ionospheric research to be considered together is commendable in this regard.6 Similar coordi- nation between NSF's magnetospheric program i n its Division of Atmospheric Sciences and its planetary mag- netospheres and atmospheres research in its Division of Astronomical Sciences would also be most welcome. Steps appropriate to each case and agency need to be taken to ensure healthy cross-disciplinary research in other areas, including comparative magnetospheres, solar wind-magnetospheric physics, and the transition- ing of research to application tools. 6See NRC, 2000, "Interim Assessment of Research and DataAnalysis in NASA's Office of Space Science," letter report, Sept. 22. THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS OPPORTUNITIES FOR SPACE MEASUREMENTS IN ENTITIES OTHER THAN NASA'S OFFICE OF SPACE SCIENCE NASA's Office of Space Science provides the most regu Iar opportunities to gain access to space through its major missions and principal-investigator-led missions, but there are other opportunities to have instrumenta- tion carried into space. One of these is payloads at- tached to the International Space Station (ISS); another is launches by DOD or its foreign partners. In this section the panel discusses issues related to these two opportu- . . n Itles. Space Station Attached Payloads The knowledge gained in studies of the interaction of the solar wind with the magnetosphere and the en- suing understanding of the entry of solar energetic par- ticles into the magnetosphere is particularly beneficial to the ISS and its occupants.7 On the other hand, the ISS is not a natural or optimum platform for observing mag- netosphere-solar wind interactions. It provides at best a limited opportunity for space physics research, owing to its orbit and facility configuration constraints. Fur- thermore, the additional qualification and safety issues pertaining to flight aboard a crowed vehicle add sig- nificantly to the cost of development, further diluting research resources. For these reasons, ISS is not a pre- ferred platform for conducting magnetospheric physics research. The panel emphasizes that continued progress in magnetosphere-solar wind interactions is of importance to ISS. The space environment plays a significant role in constraining ISS operations, as it does in constraining all space-based technology assets. Thus, continued basic research on the science of Earth's space environment is a high priority for ISS even when space physics instru- ments cannot be attached to the ISS per se. Missions of Opportunity Theabilitytooptimize the return on launch oppor- tunities by funding individual researchers to build in- struments for opportunities on non-NASA missions is an excellent concept. Nevertheless, as presently executed, it is not achieving its full potential. 7See NRC, 2000, Radiation and the international Space Station: Recommendations to Reduce Risk, National Academy Press, Wash- ington, D.C.

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS To achieve the greatest science return, adjustments need to be made in the mission of opportunity (MOO) program to increase the frequency of these opportuni- ties. The panel strongly endorses a change that would separate MOO from the SMEX and MIDEX announce- ments of opportunity, thereby allowing more frequent consideration and implementation of MOO proposals. To accomplish this change, the cost cap for MOO, in- cluding attached payloads on the ISS, should be ap- proximately halved, from $35 million to approximately $15 million. This is one mechanism whereby launches of opportunity with DOD could be more effectively le- veraged for science. (The panel recommends semian- nual considerations to provide a better match with the frequency of such opportunities and with the develop- ment times of the missions.) SCIENCE IN THE STRUCTURE OF PROJECT MANAGEMENT The principal investigator model for missions has proven highly successful in terms of science return on the investment. One important reason for this success is that science issues are given the same weight as space- craft and mission design issues. Strategic missions such as Solar Terrestrial Probes and Living with a Star mis- sions could benefit from emulating some of the manage- ment structure of these missions. A position of science manager, equal in importance to the project manager, should be established for future strategic missions. To ensure the highest quality leadership, this position should be selected competitively. The panel believes that a science consortium lead by a competitively selected PI would be another way to infuse science into the management process. INTERNATIONAL COOPERATION Historically, research in space science, especially in solar wind-magnetosphere interactions, has had a strong international element. This international element arises first from the need for globally situated, ground-based measurements and then from the immensity of the task, which requires a cooperative effort to obtain the critical mass for its successful outcome. Recently, barriers have arisen to mean i ngfu I i International cooperation. ITAR and Export Controls The International Traffic in Arms Regulations govern the export of both information and equipment that might 1 19 be used by foreign entities against the United States. All space-associated investigations are now included under these regulations, which as implemented by the State Department have placed substantial burdens on the nation's space science community. These burdens are manifested in two ways. Space physics missions have always been conducted in close collaboration with our international colleagues in Europe and Asia, primarily Japan, and in Canada. The ITAR restrictions have made it extremely difficult to continue working with these col- leagues on U.S. missions like STEREO, in which interna- tional contributions to the science payload are major elements of the design. Even rudimentary essential infor- mation concerning mission design concepts and space- craft design plans has been subject to control, making it extremely difficult, if not impossible, to involve our for- eign colleagues in making fully informed scientific judgements. It is simply impossible to properly design and build a scientific instrument without free access to relevant data on the spacecraft and mission design. The problem is even more acute in cases where instrument subsystems are provided by ou r foreign partners. One cannot team effectively if the instrument designs to which the team members contribute are sequestered. The latter point suggests the second debilitating effect that these new restrictions are having on the nation's scientific community. The tremendous return to the United States from participation in foreign missions is illustrated by the SOHO (ESA), GEOTAIL (ISAS), and CLUSTER (ESA) missions, which were implemented by foreign agencies in Europe and Japan with significant NASA instrumentation, operations, and science partici- pation. Now, however, the burdensome impact on foreign collaborating agencies has jeopardized opportunities to participate in foreign missions in the future. It is even harder to build an instrument jointly with our foreign col- laborators. Clearly, the U.S. science community would not be on an equal footing with its international colleagues had it not been able to join them in these missions. The ITAR situation is serious. Research scientists have been subjected to criminal charges and penalties. Consequently, some universities have refused to allow their researchers to accept grants and contracts with restrictive ITAR clauses. The inability to share informa- tion among partners in a mission could lead to mistakes and mission fai I u res. An amended ITAR rule was published on March 29, 2002, which applies only to university-based space re- search. The rule attempts to clarify the regulations and to remove obstacles to the conduct of university-based fundamental research in space. However, there remain

1 20 a number of serious practical problems with the new rule, including continued restrictions on which students and staff at a university can have access to information and who in partner nations can gain access. The univer- s iti es sti I I may fi n d the regu I ati ons too restri ctive and ban thei r staff from enter) ng i nto such programs. More- over, the revised statutes do not address the equally serious problem namely, that U.S. universities cannot work with the U.S. space industry without being subject to ITAR regulations. Here the restrictions are even greater than the restrictions on foreign collaborations. Information Security Presentfederal policies require all personnel having access to NASA spacecraft and science payload com- mand systems to have background security cheeks. This is enforced by ensuring that contracts with universities are consistent with NPG 281 0. This regulation requires that any individual having access to a spacecraft or its subsystems (such as science payloads) above a certain value, including the computers used to command sci- ence payloads, must be so screened. The universities are not generally convinced that they can require this of employees, especially those already hired. NASA's rul- ing means that university computer systems managers, project managers, and certain technicians and program- mers must submit to background checks as part of their i nstitution's contractual agreement with NASA on fl ight projects. However, university mission participants typi- cally have no access to spacecraft system commands or controls. Firewalls are generally placed between the external workstations from which commands are sent to the science payload and the mission operations center that sends them. The investigation teams historically as- sume responsibility for the correctness of the commands sent to their instruments on board the spacecraft, and this has not presented a security problem in the past. For a few low-cost missions, some academic institutions have assumed ful I responsibility for operations and com- manding. These missions present an information tech- nology security conundrum, for they have been extremely successful. MODELING,THEORY, AND DATA ASSIMILATION Modeling and theory need to be integrated into on- going research. Because the terrestrial magnetosphere's reconfiguration time scale is tens of minutes, far shorter than satellite orbit periods of hours to tens of hours, data sampl i ng i n Earth's magnetosphere wi 11 always be sparse THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS and i Incomplete. For th is reason, theoretical models and global simulations play a crucial role by forming a framework of understanding and context for the obser- vations. The modeling and observations need to be wed- ded closely via data assimilation in global models. This willensure that the modelsareproperlyconstrainedby the observations and can provide a suitable basis for extrapolating the observations to characterize the state and dynamics of the whole system. Because of the central role that theory and global models play, support for them needs to be robust and sustained. Global simulation codes require teams of re- searchers, each with specialized expertise in the under- lying physics, in numerical techniques, in visualization, and in user interfaces. To attract and maintain qualified researchers for efforts of this scope, the efforts cannot be supported by small (<$100,000) 3-year research grants but must be supported by larger grants (>$300,000 per year) for longer durations (5 years). It is also critical that more than one code be developed and used, because different techniques can sometimes lead to different be- havior in the simulations, and comparisons between dif- ferent codes are essential to identify consistent behavior potentially reflecting the real behavior of the system. Theory, simulation, and modeling will also become increasingly instrumental in the planning and implemen- tation of future missions. Understanding the character of the measurements required and the degree of improved understanding afforded by them and assessing the num- bers and locations of observations to most efficiently achieve definitive results will require detailed analysis with models. Mission definition and design will there- fore need to draw on the modeling resources of the community. Reliance on models will continue through- out each phase of future missions, including data analy- sis and assimilation. The dependence of future mission success on modeling underscores the need for sustained and substantial support for this effort. The increasingly integral role played by models in data analysis implies that community access to models is another aspect of the theory, modeling, and simula- tion work that needs to be supported. As discussed in the section "NOAA: Transitioning New Operational Ob- serving Platforms and Models," modeling is an area that is very appropriate for coordination with NOAA, which needs operational models. Furthermore, even in the arena of pure scientific inquiry, such coordination and community availability are important. Under NSF the GEM program has made initial strides in this direction, but making state-of-the-art models available to the com- munity remains a challenging task that requires re-

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS sources as well. Attempting to achieve this objective by requiring modeling teams to make their models avail- able is manifestly the wrong approach, since these teams are already hard-pressed to develop robust models. Rather, the effort to convert research models into community models is a separate task, which requires support for developing interfaces and computational architecture. In many ways the community model is an intermediate step in the conversion of models to opera- tional use. Support for this task should therefore derive not only from basic science, which is its primary pur- pose, but also from those agencies with an interest in developing more robust, physics-based models for pre- diction and forecasting. Support for theory and model ing is therefore a natu- ral area for interagency coordination. The benefits of modeling extend across all areas of interest, from basic science to prediction and forecasting, to mission devel- opment and planning. All of the relevant agencies- NSF, NASA, NOAA, and DOD have a vested interest in maintaining a strong theory and modeling effort, and they should find ways to coordinate their activities to ensure that support is provided in a coherent way that add resees the concerns descri bed above. TECHNOLOGY DEVELOPMENT The technological challenges for future solar-terrestrial missions are substantial and will require an effort dis- tinct from SR&T, including an SEC program similar to the Planetary Instrument Definition and Development Program of the planetary community. The primary chal- lenge for future magnetospheric missions will be meeting the need for constellation-class observations. For a new generation of spacecraft, the task is to design and develop a spacecraft architecture that can realize dramatic economies of scale even in limited production runs (tens of units). How low the ultimate cost per unit can go is not known, but the cost of the Iridium satellites, which were quite large, was ultimately reduced almost to $5 mil- lion. There are no fundamental technological reasons why a smaller platform could not be designed to cost less than this, but the task faces significant systems engi- neeri ng, management, i Integration, and testi ng problems. It is not insurmountable, however, and is the type of ambi- tious but achievable goal that should be a focus for NASA or DOD. The New Millennium Program has been suc- cessful in developing spacecraft technologies and it would seem most appropriate for it to focus some of its techno- logical investments on enabling constellation missions. 1 21 A comparable development effort is required to gain the ability to deliver tens of calibrated scientific instru- ments. Similar challenges of system engineering, man- power management, integration, and testing activities confront instrument builders contemplating the delivery of large quantities of instruments. Again, although the task is not an easy one, it does not appear to be impos- sible, and an instrument incubator program would pro- vide a mechanism to fund the long-lead-time develop- ment of instrument technologies for this purpose. For both the spacecraft and instrumentation devel- opment efforts, proper consideration must be given to an inherent feature of constellation-class missions- namely, that the large number of spacecraft and mea- surement points mitigates risk concerns and relieves the demands on instrument performance. The risk to the mission posed by the failure of a single spacecraft unit is extremely low, because the science return from, say, 45 satellites is nearly the same as that from 50. Because the redundancy is built into the constellation concept itself, one can accept single-string concepts in the spacecraft design. In a similar way, the science return is enhanced primarily by the large number of distributed measure- ments rather than by the high precision of the measure- ments, so that the requ i remeets for i nstru ment perfor- mance relative to that demanded for single-satellite missions should be critically examined. Experience with non-science-grade instrumentation strongly suggests that individual instruments performing at a much lower level can yield dramatic scientific advances when deployed in constellations. Finally, innovative and commercial solutions to spacecraft communications should be encouraged to reduce mission operations costs. Requiring the use of already overloaded systems such as the Deep Space Network for satellite tracking and communications for constel I ation missions is patently u nworkable because of the enormous operating costs that such an approach necessarily entails. Innovative, automated communica- tions approaches exist for Earth-orbiting satellites; such approaches were used very successfully for missions such as Freja and FAST and are being applied for other programs. These low-cost approaches to satellite com- munications and tracking need to be expanded aggres- sively to support constel ration missions. DATA ANALYSIS, DISSEMINATION, AND ARCHIVING The analysis, dissemination, and archiving of data acquired from NASA and non-NASA missions as well as

1 22 from ground observatories and networks are of para- mou nt i mportance to successfu I Iy ach ievi ng the science advances descri bed above. G iven that the i nterrel ated data sets to be acquired will be complex and more diffi- cult to analyze than any acquired previously, the re- sources devoted to their analysis will need to be more substantial than those for earlier missions. The man- power that needs to be brought to bear will be corre- spondingly greater, and the best way of mobilizing this expertise will be to ensure that the data are available community-wide. Data dissemination is therefore a key element of future research that advances in information technology have made much easier than in the past. The experience with missions such as ACE, SOHO, and IMAGE demon- strate that electronic dissemination of data works ex- tremely well and facilitates community involvement in their analysis. There is no reason this success cannot carry over into the next decade with equal or greater success. Given that the missions envisioned in the coming decade will not be superseded or repeated in the fore- seeable future, the preservation of their data for subsequent analysis is critically important. The standard- ization system developed for the ISTP data exemplifies the level of commonality that will be needed for these new data sets. The standardization should be extended to ground data sets as well, so that their community use can be equally widespread. Standardization is also cru- cial for preservation of the data sets. While it is expected that the distributed data systems associated with differ- ent investigators and investigations will be maintained for some period of time after the prime mission or obser- vation campaign, a centralized repository for the data will also be required and needs to be supported. It is almost certain that the number of basic issues that these data can be used to resolve will not be exhausted in the normal mission or observation lifetime of the spacecraft or the facilities used to obtain the data.8 EXTENDED MISSIONS It is widely recognized that extended missions can provide a high science return for modest additional investment, and they are strongly encouraged. The panel See NRC, 2002, Assessment of the Usefulness and Avai/abi/ity of NASA's Earth and Space Science Mission Data, National Academy Press, Washington, D.C., pp. 41~4. THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS endorses the practice of giving priority to those candi- dates for extension that most clearly support new research missions and strengthen or expand the science achieved. However, the costs of mission operations and data acquisition could be reduced considerably if track- ing and communications for extended missions could be transferred to commercial or academic institutions at the discretion of the mission PI or project management. The use of this option is consistent with the philosophy of extended missions since their prime mission objec- tives would already have been achieved, fulfilling their intended charter. If the cost of extending missions could be significantly reduced and the pressure on mission operations and data analysis resources relieved to allow more simultaneous operations, a broader array of pro- ductive observatories could be maintained for magneto- sphere-solar wind interaction science. ADDITIONAL READING A strategy for the conduct of space physics research has been set down in a number of reports by the NRC's Space Studies Board and its predecessor, the Space Sci- ence Board. These reports i nc I ude the fo I I owi ng: Space Science Board, National Research Counci 1. 1985. An Implementation Plan for Priorities in Solar-System Space Physics. National Academy Press, Washington, D.C. Space Science Board, National Research Counci 1. 1983. The Role of Theory in Space Science. National Academy Press, Washington, D.C. Space Science Board, National Research Counci 1. 1 980. Solar-System Space Physics in the 1980's: A Research Strategy. National Academy of Sciences, Washington, D.C. Space Studies Board, National Research Counci 1. 1995. A Science Strategy for Space Physics. National Academy Press, Washington, D.C. Space Stud ies Board and Board on Atmospheric Sciences and Climate, National Research Council. 1991. Assessment of Programs in Solar and Space Physics—1991. National Academy Press, Washington, D.C. The research in this field is summarized in both textbooks and conference proceedings, including the fol lowi ng:

PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS M.G. Kivelson and C.T. Russell (eds.~. 1995. Introduction to Space Physics. Cambridge U n ive rs ity P ress, N ew Yo rk. A. Nishida, D.N. Baker, and S.W.H. Cowley (eds.~. 1998. New Perspectives on the Earth's Magnetotail. Monograph 105. American Geophysical Union, Washington, D.C. B. Hultqvist, M. Oieroset, G. Paschmann, and R. Treumann (eds.~. 1999. Magnetospheric Plasma 1 23 Sources and Losses. Kluwer Academic Publishers, Dordrecht. S.l. Ohtani, R. Fujii, M. Hesse, and R.L. Lysak. 2000. Magnetospheric Current Systems. Monograph 118. American Geophysical U n ion, Wash i ngton, D.C. P. Song, H.J. Singer, and G.L. Siscoe (eds.~. 2001. Space Weather. Monograph 1 25. American Geophysical U n ion, Wash i ngton, D.C. Note added in proof: New Horizons, the first Pluto probe, has been selected as the first mission in NASA's New Frontiers program and is now in development. The probe, which will arrive at Pluto in 2015, carries solar wind plasma and energetic particle detectors in addition to its suite of remote sensing instruments and a dust experiment. In addition to its reconnaissance of the Pluto-Charon system, the probe is expected to encounter one or more Kuiper Belt objects.

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This volume, The Sun to the Earth-and Beyond: Panel Reports, is a compilation of the reports from five National Research Council (NRC) panels convened as part of a survey in solar and space physics for the period 2003-2013. The NRC's Space Studies Board and its Committee on Solar and Space Physics organized the study. Overall direction for the survey was provided by the Solar and Space Physics Survey Committee, whose report, The Sun to the Earth-and Beyond: A Decadal Research Strategy in Solar and Space Physics, was delivered to the study sponsors in prepublication format in August 2002. The final version of that report was published in June 2003. The panel reports provide both a detailed rationale for the survey committee's recommendations and an expansive view of the numerous opportunities that exist for a robust program of exploration in solar and space physics.

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