9

Report of the Panel on Solar Wind-Magnetosphere Interactions

9.1 SUMMARY OF SWMI SCIENCE PRIORITIES AND IMPERATIVES

The magnetosphere is a central part of the solar and space physics system. Its various regions interact globally in complex, nonlinear ways with each other, with the solar wind, and with the upper atmosphere. These interactions occur by a number of fundamental physical processes that operate throughout the universe. The resulting dynamic variations in the magnetospheric environment also have important practical consequences for a society that is increasingly reliant on ground- and space-based technological systems that are sensitive to Earth’s space environment.

Significant progress has been made over the past decade in achieving the science objectives identified in the 2003 decadal survey for solar wind-magnetosphere interactions.1 This progress has been achieved through a powerful combination of tools, including new data from satellites launched during the decade or just before (e.g., Cluster, IMAGE, THEMIS, TWINS); analysis of data returned from earlier missions; data from instruments flown on non-NASA operational satellites; measurements from suborbital missions and from networks of ground-based observatories; greatly improved numerical simulations; analytical theory; and laboratory work.

For the coming decade, the Panel on Solar Wind-Magnetosphere Interactions (SWMI) identified a set of eight high-priority science goals for research in solar wind-magnetosphere interactions—goals that follow naturally from the progress that has been made and will contribute substantially toward accomplishing the decadal survey key science goals for solar and space physics identified in Chapter 1 of this report. These eight goals (which are not prioritized) are as follows:

SWMI Science Goal 1. Determine how the global and mesoscale structures in the magnetosphere respond to variable solar wind forcing.

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1 National Research Council, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, The National Academies Press, Washington, D.C., 2003; and National Research Council, The Sun to the Earth—and Beyond: Panel Reports, The National Academies Press, Washington, D.C., 2003.



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9 Report of the Panel on Solar Wind- Magnetosphere Interactions 9.1  SUMMARY OF SWMI SCIENCE PRIORITIES AND IMPERATIVES The magnetosphere is a central part of the solar and space physics system. Its various regions interact globally in complex, nonlinear ways with each other, with the solar wind, and with the upper atmosphere. These interactions occur by a number of fundamental physical processes that operate throughout the uni- verse. The resulting dynamic variations in the magnetospheric environment also have important practical consequences for a society that is increasingly reliant on ground- and space-based technological systems that are sensitive to Earth’s space environment. Significant progress has been made over the past decade in achieving the science objectives identified in the 2003 decadal survey for solar wind-magnetosphere interactions.1 This progress has been achieved through a powerful combination of tools, including new data from satellites launched during the decade or just before (e.g., Cluster, IMAGE, THEMIS, TWINS); analysis of data returned from earlier missions; data from instruments flown on non-NASA operational satellites; measurements from suborbital missions and from networks of ground-based observatories; greatly improved numerical simulations; analytical theory; and laboratory work. For the coming decade, the Panel on Solar Wind-Magnetosphere Interactions (SWMI) identified a set of eight high-priority science goals for research in solar wind-magnetosphere interactions—goals that follow naturally from the progress that has been made and will contribute substantially toward accomplishing the decadal survey key science goals for solar and space physics identified in Chapter 1 of this report. These eight goals (which are not prioritized) are as follows:  WMI Science Goal 1. Determine how the global and mesoscale structures in the magnetosphere S respond to variable solar wind forcing. 1  National Research Council, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, The National Academies Press, Washington, D.C., 2003; and National Research Council, The Sun to the Earth—and Beyond: Panel Reports, The National Academies Press, Washington, D.C., 2003. 209

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210 SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY S  WMI Science Goal 2. Identify the controlling factors that determine the dominant sources of mag- netospheric plasma. S  WMI Science Goal 3. Understand how plasmas interact within the magnetosphere and at its boundaries. S  WMI Science Goal 4. Establish how energetic particles are accelerated, transported, and lost. S  WMI Science Goal 5. Discover how magnetic reconnection is triggered and modulated. S  WMI Science Goal 6. Understand the origins and effects of turbulence and wave-particle interactions. S  WMI Science Goal 7. Determine how magnetosphere-ionosphere-thermosphere coupling controls system-level dynamics. S  WMI Science Goal 8. Identify the structures, dynamics, and linkages in other planetary magneto- spheric systems. These are ambitious goals, and their accomplishment will require intelligent use of the variety of research tools that are available. Achieving some of them will require new measurements from strategic missions, either already under development (MMS and RBSP2) or to be started in the coming decade. Other contributions toward accomplishing these goals will come from community-proposed Explorer missions, suborbital flights, CubeSats, operational satellites, and instruments flown on commercial platforms as rides of opportunity. Studies that combine data from the numerous existing spacecraft will elucidate important aspects of global coupling, while analytical theory, laboratory studies, and especially powerful numerical simulations will complement the various spacecraft measurements in a synergistic attack on the science questions. However, because of budget constraints and as-yet immature technology, a number of critical investigations must be deferred to a later decade, requiring investments in the coming decade in the areas of technology innovation and development, as well as in ways to more effectively couple theory and data. In this chapter, the SWMI panel advocates a coherent and balanced program of research in solar wind-magnetosphere interactions based on a prioritized set of imperatives that will enable a cost-effective approach to accomplishing the SWMI science goals. These imperatives are sorted into three categories: (1) missions, (2) DRIVE-related initiatives, and (3) space weather. For simplicity, the panel assigned each imperative to a single primary category, although some actually apply to multiple categories. The panel prioritized the imperatives not only within but also across the three categories in order to identify the most important and most cost-effective actions to accomplish its pressing science goals. The five overall SWMI highest-priority imperatives that are developed in this chapter are the following:  WMI Imperative 1. Enhance the resources dedicated to the Explorer program and broaden the range S of cost categories. S  WMI Imperative 2. Complete the strategic missions that are currently in development (MMS, RBSP/ BARREL) as cost-effectively as possible. 2  RBSP—the Radiation Belt Storm Probes mission—was launched successfully on August 30, 2012, shortly after the release of the prepublication version of this report. The mission has since been renamed the Van Allen Probes.

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REPORT OF THE PANEL ON SOLAR WIND-MAGNETOSPHERE INTERACTIONS 211 S  WMI Imperative 3. Initiate the development of a strategic mission, like MEDICI, to determine how the magnetosphere-ionosphere-thermosphere system is coupled and responds to solar and magneto- spheric forcing. S  WMI Imperative 4. Ensure strong continued support for existing satellite assets that can still contribute significantly to high-priority science objectives. S  WMI Imperative 5. Enhance and protect support for theory, modeling, and data analysis, including research and analysis programs and mission-specific funding. Within the three categories, the panel’s full set of prioritized imperatives is as follows. The number- ing in each category reflects each priority’s relative ranking among all three categories, as shown also in Table 9.4 at the end of this chapter. 9.1.1 Missions 1. Enhance the resources dedicated to the Explorer program and broaden the range of cost categories. 2. Complete the strategic missions that are currently in development (MMS, RBSP/BARREL) as cost- effectively as possible. 3. Initiate the development of a strategic mission, like MEDICI, to determine how the magnetosphere- ionosphere-thermosphere system is coupled and responds to solar and magnetospheric forcing. 4. Ensure strong continued support for existing satellite assets that can still contribute significantly to high-priority science objectives. 9. Develop a mechanism within NASA to support rapid development, deployment, and utilization of science payloads on commercial vehicles and other missions of opportunity. NSF and DOD efforts in this regard are also encouraged. 10. Through partnership between NASA’s Heliophysics Division and Planetary Division, ensure that appropriate magnetospheric instrumentation is fielded on missions to other planets. In particular, the SWMI panel’s highest priority in planetary magnetospheres is a mission to orbit Uranus. 11. Partner with other space agencies to implement consensus missions, such as a multispacecraft mission to address cross-scale plasma physics. 15. If resources permit, initiate a strategic mission like MISTE to simultaneously measure the inflow of energy to the upper atmosphere and the response of the ionosphere-thermosphere system to this input, in particular the outflow back to the magnetosphere. 9.1.2  DRIVE-Related Actions 5. Enhance and protect support for theory, modeling, and data analysis, including research and analysis programs and mission-specific funding. 6. Ensure continuity of measurements of the upstream solar wind and interplanetary magnetic field. 7. Invest significantly in developing the technologies to enable future high-priority investigations. 8. Ensure strong multiagency support for a broad range of ground-based assets that are a vital part of magnetospheric science. 14. Strengthen workforce, education, and public outreach activities. 16. Create an interagency joint laboratory astrophysics program that addresses issues relevant to space physics.

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212 SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY 9.1.3  Space Weather 12. Encourage the creation of a complete architecture for the National Space Weather Program that would coordinate joint research, commercial, and operational space weather observations and define agency roles for producing, distributing, and forecasting space weather products. In addition the SWMI panel encourages all agencies to foster interactions between the research and operational communities and to identify funding for maintaining a healthy research-to-operations/operations-to-research program. 13. Implement a program to determine, based on past observations, the optimum set of measurements that are required to drive high-fidelity predictive models of the environment. Implementation of these imperatives will enable achievement of the exciting and high-priority science goals laid out in this report, providing a strong foundation for the accomplishment of the long-term actions described earlier in this decadal survey. To summarize, eight overarching SWMI science goals motivate sixteen prioritized, actionable imperatives that are required to enable the goals (these prioritized impera- tives and their mapping to decadal categories from Part I are shown in Table 9.4 at the end of this chapter). 9.2  INTRODUCTION TO SWMI SCIENCE This section gives a brief introduction to the magnetosphere and its interactions with the solar wind and the upper atmosphere. This information provides a context for the subsequent discussion of the past decade’s accomplishments and important unanswered questions, leading to the SWMI panel’s science goals for the coming decade and the initiatives necessary to accomplish them. 9.2.1  What Is the Magnetosphere? The magnetosphere (Figure 9.1) is a vast, highly coupled system governed by fundamental physical processes and characterized by complex, nonlinear linkages between its different parts. It is formed by the interaction of the solar wind plasma stream and its embedded magnetic field with Earth’s intrinsic magnetic field. Earth with its field is an obstacle in the solar wind flow, carving out a separate plasma domain where Earth’s field has dominant control over the motions of the electrically charged particles trapped there. These charged particles come from both the solar wind and Earth’s upper atmosphere. This region of dominance, the magnetosphere, extends out to approximately 10 Earth radii on the sunward side of Earth and, in a long “magnetotail,” extends to well beyond the Moon on the side away from the Sun. The shape of the magnetosphere is determined by the balance between the pressure exerted by the solar wind plasma and interplanetary magnetic field (IMF) and the pressure of Earth’s plasma and magnetic field. Earth is only one of six of the Sun’s planets (Mercury, Earth, Jupiter, Saturn, Uranus, and Neptune) that are known to have a magnetosphere by virtue of their intrinsic magnetic fields. Ganymede, one of Jupiter’s satellites, also has its own tiny magnetosphere embedded within Jupiter’s giant one. 9.2.1.1 Regions The magnetosphere is made up of regions with different plasma characteristics. As illustrated in Figure 9.1, the shape of the underlying geomagnetic field lines governs the morphology of these various regions. Nearest Earth, there is a relatively cold and dense region called the plasmasphere. The plasma- sphere contains plasma that has escaped from the ionosphere, the ionized region of Earth’s upper atmo- sphere. Coincident with the plasmasphere or residing at slightly larger radial distances are higher-energy charged-particle populations called the ring current and radiation belts. Ring-current particles drift azi-

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REPORT OF THE PANEL ON SOLAR WIND-MAGNETOSPHERE INTERACTIONS 213 FIGURE 9.1  A schematic diagram of the magnetosphere, with various regions indicated. SOURCE: Adapted from L.A. Weiss, P.H. Reiff, R.V. Hilmer, J.D. Winningham, and G. Lu, Mapping the auroral oval into the magnetotail using dynamics explorer plasma data, Journal of Geomagnetism and Geoelectricity 44(12):1121, 1992. Figure 9-1 muthally around Earth because of the strong magnetic field gradients, with positively charged ions drifting west and electrons drifting east, producing an electrical current large enough to substantially modify the global magnetic field. At still higher energies, the similarly drifting radiation-belt population possesses tre- mendous energy per particle, but because of its very low density carries little net current. Beyond the ring current and radiation belt is a low-density, hot plasma called the plasma sheet, extending to large distances into the magnetotail and serving as the reservoir for much of the plasma that ultimately feeds the inner- magnetosphere ring-current and radiation-belt populations. The high-latitude magnetic flux regions that are nearly devoid of plasma and that extend down the magnetotail from the polar caps are called the tail lobes. The magnetopause is the boundary of the magnetosphere, which separates it from the surrounding regions dominated by the solar wind and its magnetic field. Upstream from the magnetopause there is a standing shock wave, the bow shock, at which the supersonic solar wind is slowed and heated, enabling it to flow around the magnetospheric obstacle. The transition region of shocked solar wind between the bow shock and the magnetopause is known as the magnetosheath. A final region, beyond the bow shock, is called the foreshock, in which fast particles move upstream along the IMF and perturb the supersonic flow of the incoming solar wind. Another term that is often used synonymously with the magnetosphere- ionosphere system is “geospace.” 9.2.1.2  Physical Processes Several primary physical processes produce the rich phenomenology of the structured, time-dependent magnetospheric system. “Magnetic reconnection” is the process by which energy stored in magnetic fields

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214 SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY is converted to plasma thermal energy, plasma bulk flow energy, and energized particles through a topo- logical magnetic field reconfiguration. Charged particles in general conserve certain quantities of motion called adiabatic invariants, which relate aspects of the particle motion to magnetic field parameters and can lead to reversible energy changes. The term “wave-particle interactions” (WPIs) refers to the general and broad process by which electromagnetic waves and charged particles exchange energy and momentum. “Turbulence” describes the flow state of a fluid (including magnetized fluids) that is chaotic and stochas- tic. Turbulent media provide a rich opportunity for WPI. These same physical processes, determining the structure and dynamics of Earth’s coupled solar wind-magnetosphere system, also govern other planetary magnetospheres; by studying these fundamental processes in our own neighborhood, researchers glean universally applicable knowledge. 9.2.1.3  Coupling to the Ionosphere and Solar Wind The magnetosphere is physically bounded by the ionosphere and solar wind at its lower and upper extents, respectively. Earth’s magnetosphere has no internal plasma sources, and so these boundary regions are the two major sources of magnetospheric plasma. The solar wind is a large source of protons and electrons into the magnetosphere, while the ionosphere contributes not only protons but also heavy ion species like oxygen, helium, and nitrogen, and their accompanying electrons. Furthermore, the solar wind and IMF, through magnetic reconnection and viscous interaction, drive convective flow throughout the magnetosphere. The ionosphere, with its high conductance, regulates and modulates this convective flow. In addition, the neutral gas of the upper atmosphere, known as the thermosphere, can also influence magnetospheric flow through ion-neutral collisions. 9.2.1.4  Space Weather and the Magnetosphere “Space weather” is the name given to the time-dependent conditions and changes that occur in near-Earth space to the magnetospheric plasmas and fields. These include changes in the plasma density, temperature, and spatial distributions, from the cold plasmasphere to the very energetic radiation belts. In particular, space weather implies changes that have significant impact on technology and society. For example, the variable ionosphere and plasmasphere alter geolocation signals from GPS and transmissions from communication spacecraft; strong magnetospheric currents create geomagnetically induced currents in power distribution systems; energetic particles cause radiation damage to microelectronics and space- farers; and substorm-related satellite charging causes malfunctions and surface degradation. 9.2.1.5  Magnetospheric Questions That Flow from the Motivations The motivations underlying the study of solar and space physics3 apply directly to the study of Earth’s magnetosphere and its interaction with the solar wind and upper atmosphere: Earth’s magnetosphere (and those of other planets) is a fascinating, complex system, in which fundamental physical processes that operate throughout the universe combine with unique conditions of plasma sources, sinks, and drivers to create dynamic conditions that can affect humans and the technologies they depend on in space and sometimes on the ground. 3  See the introduction to Part II of this decadal survey report.

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REPORT OF THE PANEL ON SOLAR WIND-MAGNETOSPHERE INTERACTIONS 215 • Motivation 1. Understand our home in the solar system. The fundamental motivation of wanting to understand the fascinating Sun-Earth system drives the desire to learn what determines the dynamically changing charged-particle environment of the magnetosphere, from the lowest-energy particles emerging from the upper atmosphere, to the hazardous high-energy particles of the radiation belts. What exactly are the relationships between the different magnetospheric regions, and how are these regions coupled to the solar wind and upper atmosphere? How do these linkages determine magnetospheric dynamics? And what new insights do researchers gain from examining the similarities and differences in these regions and processes at other planets? • Motivation 2. Predict the changing space environment and its societal impacts. A thorough under- standing of this complex and highly coupled system will make it possible to predict its behavior under a variety of changing conditions, allowing anticipation and recognition of conditions that are adverse to human life and technology. Within the magnetosphere, researchers are particularly interested in how they can evaluate and predict damaging particle populations and electrodynamic fields. • Motivation 3. Reveal universal physical processes. The geospace system constitutes a laboratory for exploring a wide variety of processes that operate throughout the universe. These include the ways in which ionized outflows can be driven from planetary atmospheres; how waves and particles provide coupling between disparate plasma regions; how plasmas interact with neutral materials; how magnetic reconnection occurs and how it energizes particles; how collisionless shock waves work; and how plasma turbulence is generated and dissipated and affects other dynamical processes. The following sections outline recent progress made toward addressing the fundamental questions raised by these motivations; indicate the outstanding problems where significant progress can be accom- plished in the near future, leading to the identification of specific science goals for the coming decade; and lay out the SWMI panel’s imperatives for actions that are needed to meet those goals. 9.3  SIGNIFICANT ACCOMPLISHMENTS OF THE PREVIOUS DECADE 9.3.1  Scientific Progression The science of solar wind-magnetosphere interactions was established near the dawn of the space age, only 50 years ago. Since that time, knowledge of the magnetosphere has grown tremendously, progressing from discovery to understanding. Early research focused on the discovery and exploration of the different regions that lie above Earth’s atmosphere: the radiation belts, the plasmasphere, the plasma sheet, and the solar wind. As researchers discovered these regions and the particle populations that define them, they sought to identify and under- stand the physical processes that accelerate and transport the particles. They also learned that the system is not static, but rather exhibits substantial variability with identifiable patterns. It became clear that differ- ent regions are not independent, but rather are intimately linked, and the variability and processes in one region can be clearly related to those in other regions. Scientists now appreciate that the magnetosphere is a vast, highly coupled system that involves a wide array of fundamental physical processes and complex linkages between different regions. In the past decade this system’s impact on human technology and society has also come to be appreciated: storms and other disturbances in geospace have significantly disrupted and disabled spacecraft and ground-based power grids. Finally, as mankind has reached out to explore the solar system, researchers have discovered that other solar system bodies possess magnetospheres that exhibit many of the same processes found near Earth, but frequently manifested in different ways, and producing very different structure and dynamics.

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216 SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY System Dynamics and Consequences Understanding PredicƟon We are here! Space Weather Effects Comparison Linkages with other Universal systems Processes Regions Discovery Ɵme FIGURE 9.2  A schematic of the temporal progression of scientific research into solar wind-magnetosphere interactions, with time increasing upward. Figure 9-2 Thanks to observations from new satellite missions, new analyses of data from previous missions, and improved numerical modeling capabilities, substantial progress has been made in the past decade to advance the field along the progression illustrated in Figure 9.2. Here the SWMI panel reviews some of these significant accomplishments. This review is by no means comprehensive, but it serves to illustrate the progress that continues to be made across a broad front of scientific endeavors. These accomplish- ments lay the foundation for what needs to be done in the future to continue the progression toward a comprehensive and actionable understanding of the SWMI system. 9.3.2 Regions 9.3.2.1  Statistical Description of Magnetospheric Properties The decades-long reconnaissance of magnetospheric structure and dynamics has culminated in new statistical descriptions of plasma properties in the magnetosphere, particularly their systematic variation in response to solar wind drivers. For example, use of data sets that extend over more than a solar cycle showed the activity-dependent spatial distribution of plasma-sheet fluxes to be well described by a rela- tively simple convection model coupled with losses. These and similar results have solidified the statistical picture of the structure of the inner magnetosphere, but they do not capture the dynamic evolution of these regions, nor the coupling with the rest of the system. 9.3.2.2  Measuring Invisible Populations During the past decade, several new techniques were exploited for measuring the plasma density over more extended regions than allowed by single-point, in situ measurements. On the ground, observations of field-line resonances enabled instantaneous determination of the equatorial plasma density over a broad spatial domain. In space, active radio sounding techniques probed the density variation along magnetic

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REPORT OF THE PANEL ON SOLAR WIND-MAGNETOSPHERE INTERACTIONS 217 field lines, and both radio sounding and extreme ultraviolet (EUV) images revealed not only the dynamics of the boundaries of the plasmasphere, but also actual global plasmaspheric density distributions. These advances made it possible for the first time to observe the global evolution of the plasmasphere in response to variable driving of the magnetosphere. Energetic neutral atom (ENA) imaging established itself as a valuable tool in determining the global- scale configuration, dynamics, and composition of the ring current (e.g., see Figure 9.6 below). It was found that the peak of the ring-current proton distribution during the main phase of magnetic storms could lie not in the historically expected afternoon location, but in the early morning sector, revealing that the coupling with the ionosphere can very strongly alter the behavior of magnetospheric plasmas. In addition, the hydrogen component of the ring current builds up and decays gradually throughout a magnetic storm, but the oxygen component rises and falls impulsively. These variations in oxygen are often correlated with substorm injections, highlighting the coupling between the magnetosphere and ionosphere. These results represent more than a huge leap forward in system-level knowledge of the ring current; they also offer a tantalizing hint of the dynamic magnetospheric behavior that could be uncovered with higher-resolution, continuous, and global imaging. 9.3.3 Processes 9.3.3.1  Magnetic Reconnection The recent decade has witnessed substantial progress in understanding how magnetic reconnection works. For example, increased computing power has allowed full-physics simulations describing the essential physics and structure of the diffusion region, the key region where magnetic field lines break and reform. The decoupling of ion and electron motions plays a key role, accelerating the energy release, creating high-speed electron beams, and warping the magnetic field. These predictions have facilitated the first direct detections of the ion diffusion region in the magnetosphere (Figure 9.3) and in the labora- tory, as well as glimpses of the much smaller electron diffusion region. The observations in the vicinity of the diffusion region revealed surprisingly that electrons can be accelerated by reconnection to hundreds of kiloelectron volts. The past decade has also witnessed surprises regarding the triggering and modulation of reconnection: theoretical studies using fully three-dimensional simulations revealed that the added dimension facilitates plasma instabilities that can disrupt the diffusion region, making reconnection highly turbulent. Obser- vationally, reconnection seems to behave differently in different regions. On the dayside magnetopause, as well as recently discovered in the solar wind, magnetic reconnection can be quite steady in time and extended in space. In the magnetotail, however, reconnection is most often patchy and bursty, producing narrow flow burst channels. Using multispacecraft observations, these reconnection-generated flow chan- nels have now been demonstrated to initiate magnetospheric substorms. 9.3.3.2  Wave-Particle Interactions A delicate balance between acceleration and loss caused by wave-particle interactions controls the variability of radiation belt fluxes during geomagnetic storms. Understanding of magnetospheric plasma waves and their role in radiation belt dynamics has increased significantly during the past decade. Statisti- cal analyses of satellite wave data have led to the development of global models of the wave environment. These have then been used to quantify the rates of energization and scattering loss to the atmosphere. Time-dependent two-dimensional and three-dimensional models for the radiation belts and the ring cur-

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218 SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY magnetopause Polar Bow opa use magnetotail Wind et shock agn M Hall By By (nT) By Hall By Magnetotail (nT) Solar wind Bz (nT) Vx Hall Ex (km/s) Ex (mV/m) By/B0 4 0.4 Z (di) 2 0.2 0 0.0 Wind -0.2 -2 -0.4 -4 -10 0 10 Polar X (di) FIGURE 9.3  At the center is a schematic of the magnetosphere indicating the regions where magnetic reconnection usually occurs. The lower panel is a simulation result of the quadrupolar magnetic field topology in the ion diffusion region, and the left and right panels are observations of these field topologies by the Polar and the Wind spacecraft, respectively. For comparison, the relative motions of the two spacecraft through the reconnection region are indicated on the simulation result panel. The Hall field arises from the decoupling of the ions from the electrons. SOURCE: Left: Adapted from F.S. Mozer, S.D. Bale, and T.D. Phan, Evidence of diffusion regions at a subsolar magnetopause crossing, Physical Review Letters 89:015002, 2002. © 2002 The American Physical Society. Middle, upper: C. Day, Spacecraft probes the site of magnetic reconnection in Earth’s magnetotail, Physics Today 54:10, 2001. Middle, lower: Adapted from T.D. Phan, J.F. Drake, M.A. Shay, F.S. Mozer, and J.P. Eastwood, Evidence for an elongated (>60 ion skin depths) electron diffusion region during fast magnetic reconnec- tion, Physical Review Letters 99:255002, 2007. © 2007 The American replaced Figure 9-3 Physical Society. Right: Adapted from M. Øieroset, T.D. Phan, M. Fujimoto, R.P. Lin, and R.P. Lepping, In situ detection of collisionless reconnection in the Earth’s magnetotail, Nature 412:414-417, 2001, doi:10.1038/35086520. rent, which incorporate the effects of wave-particle scattering, have been developed and are beginning to provide a more realistic picture of storm-time particle dynamics. Satellite observations of peaks in the radial profiles of radiation-belt electrons have demonstrated that local acceleration due to WPI may at times dominate over traditionally accepted acceleration associated with diffusive radial transport. Diffusive radial transport may actually lead to enhanced losses at the mag- netopause, causing a decrease in trapped flux rather than an increase. Particle interactions with “chorus” waves in particular have been shown to provide a major probable source of local acceleration. The source of plasmaspheric hiss, another wave mode known to be responsible for strong losses of radiation belt elec- trons near the edge of the plasmasphere, has been shown to be discrete chorus emissions generated in the low-density region outside the plasmapause (Figure 9.4). Chorus emissions have also been shown to be the dominant cause of scattering of plasma sheet electrons, leading to their precipitation into the atmosphere, where they produce the diffuse aurora. Intriguingly, recent observations of extremely large-amplitude waves suggest that nonlinear wave-particle physics may play an important role in radiation belt dynamics. 9.3.3.3 Turbulence In the past decade the presence of large-amplitude MHD fluctuations in the magnetotail plasma sheet has been established by spacecraft measurements, and the spatial scales (~1 RE) and timescales (~1 minute)

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REPORT OF THE PANEL ON SOLAR WIND-MAGNETOSPHERE INTERACTIONS 219 ψ0/ψres –0.71 –0.48 –0.24 0 –0.24 –60º –40º –20º 0º 20º ψ0 Day side Night side 3 30º 20º 2 1 10º Earth radii, RE 0 0º –1 Plasmasphere –2 –3 FIGURE 9.4  Ray path calculations showing how discrete whistler-mode chorus emissions generated outside the plasma- pause can be refracted into the plasmasphere, become trapped, and eventually merge to form incoherent plasmaspheric hiss. SOURCE: J. Bortnik, R.M. Thorne, and N.P. Meredith, The unexpected origin of plasmaspheric hiss from discrete chorus emissions, Nature 452:62-66, 2008, doi:10.1038/nature06741. of the fluctuations have been determined. Global simulations of the solar-wind-driven magnetosphere are reaching high-enough spatial resolutions in the magnetotail to enable them to predict irregular vortical flows at these scales and to statistically match the properties of the vortical flows to observed fluctuations. However, the dynamical nature of these fluctuations, what causes them, and their influence on the behavior of the magnetosphere have not been determined. Fluid turbulence provides one pathway by which energy moves across scale sizes from large to small where energy can be dissipated in the form of heating. When and where turbulent processes play a significant role in magnetospheric dynamics remain unclear. 9.3.4 Linkages 9.3.4.1  Coupling with the Solar Wind The variable solar wind drives a wide range of variations in magnetospheric behavior. Over the past decade, continuous measurements of the solar wind combined with observations throughout geospace have enabled significant advances in identifying which specific large- and mesoscale solar-wind proper- ties produce different modes of magnetospheric response (e.g., storms, steady magnetospheric convection events, sawtooth events). For example, studies have linked variations in the solar wind dynamic pressure to radiation-belt loss and energization processes. Other studies quantified that the strength of geomagnetic storms depends on both the electrodynamic coupling between the solar wind and the magnetosphere and plasma loading of the magnetosphere, including both ionospheric and solar wind sources. Spacecraft observations and numerical simulations reveal that solar-wind plasma entry into the magnetosphere is surprisingly efficient under “quiescent” conditions of northward interplanetary magnetic field. This plasma in turn participates as a substantial element of the storm-time ring-current development when southward interplanetary magnetic fields couple with and energize the magnetosphere. Additional progress has also been made in delineating the effects of smaller-scale solar wind variations on magnetospheric behavior. While some of the specific processes that mediate this coupling with the solar wind were clarified in the past decade, major questions remain regarding the spatial extent over which they operate and the condi-

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250 SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY FIGURE 9.14  The magnetic dipole axis of Uranus is strongly tilted with respect to its rotational axis, which in turn lies near the orbital plane. Thus, depending on the season, the effects of solar wind-magnetosphere interaction vary dramatically over the course of each day. Uranus significantly expands the parameter range over which scientists can study magnetospheric structure and dynamics. SOURCE: Courtesy of Jerry Goldstein, Southwest Research Institute. Figure 9-14 in UV, IR, and ENA. To make continued exciting progress toward understanding other magnetospheres, the SWMI panel concluded with the following SWMI imperative: SWMI Imperative: Through partnership between NASA’s Heliophysics Division and Planetary Divi- sion, ensure that appropriate magnetospheric instrumentation is fielded on missions to other planets. In particular, the SWMI panel’s highest priority in planetary magnetospheres is a mission to orbit Uranus. 9.5.2.5  Future Strategic Missions Determining how mesoscale and global structures in the magnetosphere respond to variable solar wind forcing and understanding how plasmas interact within the magnetosphere and at its boundaries both require observations that match these scales. This effort essentially necessitates global-scale imaging

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REPORT OF THE PANEL ON SOLAR WIND-MAGNETOSPHERE INTERACTIONS 251 and multisatellite constellation missions to obtain measurements of the physical parameters necessary to accomplish these science objectives. A MEDICI-like mission will address these objectives through global imaging, which is technically feasible primarily for the inner and middle magnetosphere. For the outer magnetosphere, which forms the primary solar-wind entry and energy-storage region, the highly tenu- ous plasma makes “imaging” by means of local measurements on many spatially separated satellites the best approach to resolving global and mesoscale structure. Below, some exciting mission concepts are highlighted that can address these challenging objectives but will require technology developments in the coming decade to achieve feasible cost and readiness levels. Magnetospheric Constellation Mission (MagCon) Science Goals. Understanding the mass and energy transport at global and mesoscales in Earth’s magneto- spheric plasma sheet and reconnection regions of the near-Earth magnetotail, plus the dayside and flanks of the magnetopause and bow shock regions, can be implemented using a multisatellite in situ mission such as the MagCon mission. The prime overarching objective of the mission is to determine how the mag- netosphere stores, processes, and releases energy in the magnetotail and accelerates particles that supply the inner magnetosphere’s radiation belts. It would track the spatial-temporal plasma structures and flows associated with the solar wind plasma entry across the magnetopause and transport within and through the magnetotail. On the dayside and flanks the constellation would provide multipoint measurements of the upstream solar wind input, and the response throughout the magnetosphere, enabling determination of how the entire system responds to variable solar wind driving. In the magnetotail it would provide a map of the global plasma flows and field configurations, leading to determination of whether they are internally developed or externally triggered. Throughout the mission, MagCon would provide a global “picture” of these otherwise invisible regions of the magnetosphere. Mission Concept. MagCon uses many satellites separated by mesoscale distances (~1-2 RE) that make magnetic field plus plasma and energetic particle distribution function measurements at multiple points simultaneously with relatively rapid cadence. The mission requires a significant number of spacecraft, 36 in the concept the SWMI panel evaluated, to achieve mesoscale spacing while filling a significant fraction of the near-Earth space using orbits with perigees in the 7-8 RE range and apogees dispersed uniformly up to 25 RE with low inclination. The satellites would be simple ~30-kg-class spin-stabilized vehicles with their spin axes perpendicular to the ecliptic. Each spacecraft would carry a boom-mounted fluxgate magnetometer, a three-dimensional ion-electron plasma analyzer, and simple energetic ion-electron particle telescopes. To implement such a constellation requires development of small satellite systems and instruments that can be more cheaply manufactured and tested in a reasonable time frame (2-3 years) with acceptable reliability levels, plus a better match between launch vehicle capabilities and constellation mission needs. Magnetospheric Constellation and Tomography (MagCat) Science Goals. With science objectives similar in many respects to those of MagCon, MagCat would address some of the most critical processes in Sun-Earth connections: plasma entry into the magnetosphere, plasma-sheet formation and dynamics, and investigation of bow-shock structure, plasmaspheric plumes, and other mesoscale structures that form in response to solar-wind variability. To achieve this objective requires observations with a minimum spatial resolution of 0.5 RE at a minimum time cadence of 15 s. MagCat could provide those required measurements. Mission Concept. MagCat is a 20-spacecraft mission that would provide a combination of two-dimensional images of the equatorial outer magnetosphere and multipoint in situ observations made concurrently and in

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252 SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY the same imaged region. The spacecraft would be in two coplanar orbits that pass through critical regions in the magnetotail, flank, and subsolar magnetosphere. Each satellite would transmit radio waves to all others, obtaining 190 line-of-sight densities, enabling tomographic images of plasma density over large regions with an average spatial resolution of 0.32 RE at 12 s cadence. Each satellite would carry a suite of plasma and field instruments that provide complementary in situ data throughout the imaged area for ground truth as well as revealing the detailed plasma processes in the region. The nominal payload would include a 3-axis fluxgate magnetometer, electrostatic analyzers that measure three-dimensional ion and electron distributions, a relaxation sounder to determine the ambient density, and a radio tomography instrument. As for MagCon, the pre-CATE estimate for the MagCat mission was deemed beyond the scope of the budget in the coming decade. Thus, there is a similar need in the coming decade to develop cost-effective and efficient manufacturing procedures to mass produce a large number of spacecraft and instruments. 9.5.2.6  International Partnerships International partnerships involving a consortium of individual space agencies can be an effective way to pool limited resources to achieve an outstanding science goal whose importance is agreed upon by a consensus of these agencies. For example, determining the cross-scale coupling physics involved in key plasma processes is believed to be crucial for complete understanding of the causes and consequences of these processes. None of the past, current (e.g., Cluster and THEMIS), or planned missions (e.g., MMS) are designed to address the cross-scale aspects of these processes. However, a mission concept has been developed in Japan, Canada, and Europe that involves a fleet of spacecraft performing simultaneous in situ measurements at electron, ion, and fluid scales. Such a mission can investigate how turbulence transports and dissipates energy over multiple scales and how kinetic microscale instabilities are modulated by mac- roscale properties of the plasma, as well as the relative role of global conditions versus microscale physics in determining the structure and dynamics of magnetic reconnection. These are all important aspects of SWMI critical science goals 6 and 7. This and other international, cross-agency partnerships should be pursued when available and possible. SWMI Imperative: Partner with other space agencies to implement consensus missions, such as a multispacecraft mission to address cross-scale plasma physics. 9.5.3  DRIVE-Related Actions In this section, the SWMI panel expands on a number of issues that have a material impact on the national ability to conduct an effective and productive solar and space physics research effort. 9.5.3.1  Solar Wind Monitor Knowledge of upstream solar wind conditions, the interplanetary magnetic field, and solar energetic particles is required in essentially all of the programs that would address the SWMI science objectives. Currently, instruments on the ACE8 spacecraft, which orbits around the L1 libration point approximately 1.5 million km from Earth, provide these data. Follow-ons to ACE, which could also be instrumented with a solar coronagraph to view Earth-bound coronal mass ejections, are needed both to satisfy SWMI science goals and as part of a space weather forecasting system (see Chapter 7). The SWMI panel does not have 8  Information about the Advanced Composition Explorer (ACE) is available at http://www.srl.caltech.edu/ACE/.

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REPORT OF THE PANEL ON SOLAR WIND-MAGNETOSPHERE INTERACTIONS 253 a preference for which agency conducts this mission, but these data must be available on a continuous basis throughout the coming decade. SWMI Imperative: Ensure continuity of measurements of the upstream solar wind. 9.5.3.2  Theory, Modeling, and Data Analysis Efforts related to advancing theoretical and modeling studies of geospace are another key component of the solar and space physics research effort. The future of data analysis is becoming ever more closely linked with modeling, and tools need to be developed to enable a broad segment of the community to access and combine observations and model results. Currently, a great deal of the science return from NASA missions and NSF ground-based measurements occurs through theory, modeling, and data analysis sup- ported in a portfolio of both NASA and NSF grants programs. The high productivity and cost-effectiveness of these programs argue strongly that major increases in science output can be achieved with modest funding increases. The SWMI panel therefore strongly encourages the agencies to enhance the funding of R&A programs. In a constrained funding environment, this is one of the most cost-effective ways to ensure that the high-priority science objectives outlined in this decadal survey will be accomplished. In addition, the panel supports funding enhancements to enable the creation of a new program within NASA’s research portfolio to bring critical-mass multidisciplinary science teams together to address grand challenge problems. However, these proposed Heliophysics Science Centers must not be created at the expense of the current research programs, e.g., the Heliophysics Theory program. They must also be com- peted regularly and be focused on addressing a key science question. The SWMI panel also strongly affirms the vital role that theory and modeling supported by mission funding play in advancing the science objectives of the missions. The panel believes that this role should be protected by requiring external reviews of the impacts of any theory-funding reductions on the ability of a mission to fulfill its science objectives. SWMI Imperative: Enhance and protect support for theory, modeling, and data analysis, including research and analysis programs and mission-specific funding. 9.5.3.3  Innovation and Technology In reviewing the white papers submitted from the community, the SWMI panel found that many relied on tried-and-true measurement techniques. However, while much can be accomplished with new applications of state-of-the-art technology, it is also clear that to accomplish the most challenging science objectives of the future, the development of new, innovative concepts will be necessary. Indeed, in an observation-driven field such as solar and space physics, it is often new observations that point the way to the science questions of the future. Advanced instrumentation for constellation missions is an area of particular need for ultimate realiza- tion of the science goals the SWMI panel has outlined. Targeted research on how to acquire, analyze, and display the simultaneous measurements from many spacecraft is also needed, along with development of new ways to integrate global modeling and global observations. Another area where technology development efforts could have a big scientific payoff is global imaging systems, particularly systems that can stand off, well outside the magnetosphere’s bow shock, and image its boundary structures using a range of techniques such as neutral atom imaging (ENA), scattered light, and X-ray emission. New instrumentation with increased sensitivity and resolution is needed to achieve spatial

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254 SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY resolutions (~0.5 RE) and image cadences (~15 min) sufficient to image the boundaries and visualize their motions and structure. New techniques for reducing background noise are also needed. A number of the science goals outlined in Section 9.4 involve connecting phenomena and signatures that occur in the ionosphere with their corresponding phenomena and signatures in the magnetosphere. Because this connection occurs primarily via the geomagnetic field, which is strongly distorted and highly variable due to currents flowing within the magnetosphere itself, an accurate mapping between the iono- sphere and magnetosphere for all relevant conditions is lacking. Current efforts to relate magnetospheric and ionospheric physics thus typically rely on empirical models based on statistical analysis of large data sets acquired over long time periods and under a variety of conditions. Techniques to definitively establish the instantaneous mapping are thus urgently needed. It is the SWMI panel’s view that the current limited investment in technology development has discour- aged the development of new instrument concepts. Therefore, the panel strongly supports the creation of a robust Heliophysics Instrument and Technology Development Program (HITDP) in the context of the NASA ROSES. In the panel’s opinion, this program should be funded sufficiently to support several grants at levels significantly larger than is possible at present through supporting research and technology. Moreover, the missions of the future will require new satellite and systems technologies. Such technolo- gies are most appropriately pursued by the Office of the Chief Technologist, in close consultation with the Heliophysics Division so that real mission needs are addressed. Possible technology development needs noted by the panel include, but are not limited to, techniques that make it feasible to produce and deploy large numbers of identical spacecraft at an affordable price and with high radiation tolerance; solar sails; advanced propulsion and power; low-cost launch vehicles; mass-production techniques; component min- iaturization; and wireless communications within a satellite. Another area where near-term investments will help reduce the risk and long-term cost of new instru- ments is in raising the technology readiness level (TRL) of the instruments of the future, particularly in the area of providing in-space operating experience. The panel believes that suborbital flights are cost-effective ways to mature instrument technologies for future science applications, even if no immediate science can be obtained from flying suborbitally. Thus, while the panel applauds and supports the science output of the suborbital program, it also encourages the utilization of suborbital flights for increasing the TRL of instruments with long-term science applications, but not necessarily science output from the flight itself. SWMI Imperative: Invest significantly in developing the technologies to enable future high-priority investigations. 9.5.3.4  Ground-Based Instrumentation Since the science objectives established by this panel emphasize a global view of the coupled mag- netosphere-ionosphere-thermosphere system the SWMI panel strongly supports networks of ground-based instruments. For example, the MEDICI mission will be greatly enhanced by the data from magnetometer arrays and the SuperDARN network, and both MISTE and MEDICI benefit from the data provided by incoherent scatter radars (ISRs) and networks of all-sky cameras. The panel concluded that existing facili- ties, e.g., ISRs, SuperDARN, and magnetometer arrays, should continue to be supported, upgraded, and perhaps enhanced. Moreover, research drawing on increasing access to data from these instrument suites via the Internet would greatly benefit from the development of standards for data collection and access. The SWMI panel takes note of successful NASA contributions in the past decade to ground-based assets that directly support their space missions, e.g., the THEMIS all-sky camera network, for which an example image is shown in Figure 9.15. The panel encourages continuation and expansion of these efforts. The

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REPORT OF THE PANEL ON SOLAR WIND-MAGNETOSPHERE INTERACTIONS 255 FIGURE 9.15  Composite auroral image constructed from the THEMIS All-sky Imager network. SOURCE: Courtesy of NASA/ Goddard Space Flight Center Scientific Visualization Studio; available at http://www.youtube.com/watch?v=GEebRsRnwm0. panel also notes that DOD supports a number of ground-based observatories that contribute significantly Figure 9-15 replaced to the science objectives outlined above and encourages continued sponsorship of those facilities as well. The panel further supports negotiation of international agreements to enable coordination and collabora- tion with non-U.S. ground-based capabilities. SWMI Imperative: Ensure strong multiagency support for a broad range of ground-based assets that are a vital part of magnetospheric science. 9.5.3.5  Laboratory Studies The SWMI panel endorses the recommendation in the plasma science decadal survey that the Depart- ment of Energy be the prime steward for laboratory plasma science. However, to enable research on basic plasma physics that will be of greater utility to SWMI science objectives, the panel supports the creation of an interagency joint laboratory astrophysics program. This program should be competed on a regular basis, include selection criteria that focus on issues relevant to space physics, contain a mechanism for outside investigators to have access to supported facilities, and be open to proposals from any institution. SWMI Imperative: Create an interagency joint laboratory astrophysics program that addresses issues relevant to space physics.

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256 SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY 9.5.3.6  Education and Workforce At the most basic level, the field of solar and space physics needs a robust, well-trained, and talented workforce to accomplish its science goals, as well as an educated populace that recognizes the value of addressing these exciting scientific challenges. The SWMI panel therefore endorses curriculum develop- ment efforts across all academic levels, as well as faculty development programs. The panel notes in particular the success of the NSF Faculty Development in Space Science program and strongly supports its continuation and enhancement throughout the coming decade. The panel further endorses funded training opportunities for both undergraduate and graduate students, especially for participation in the development of flight hardware. Such opportunities could be provided by science grant augmentations, stand-alone education and public outreach grants, and mission-related funding. SWMI Imperative: Strengthen workforce, education, and public outreach activities. 9.5.4  Space Weather As suggested by its title, the long-term goal of this decadal survey is to have the knowledge to ensure the well-being of a society dependent on space. Actionable knowledge of space environment effects involves the ability to characterize conditions anywhere in the system at any time in the past, as well as to predict future conditions with good fidelity. This capability requires an understanding of the full, coupled solar-terrestrial system that encompasses all the regions, processes, and coupling described above, across spatial scales from meters to hundreds of Earth radii. It includes understanding the fundamental microscopic physics as well as the global system behavior in response to variable driving. The ultimate objective in the study of solar wind-magnetosphere interactions is to know how solar and solar-wind input at various spatial and temporal scales determines the nature and behavior of magnetospheric populations, structures, and processes and to be able to predict those that have significant space weather impacts. There are three aspects of accomplishing this long-term goal: 1. Establishment of the foundation of comprehensive scientific understanding; 2. Development of sound, validated space environment models; and 3. Fielding of the optimum operational assets to drive those models. The scientific program presented above will put in place some of the tools essential to achieving this vision, particularly by defining outstanding questions that still inhibit a comprehensive scientific understanding. The development of sound, validated space environment models requires a healthy research-to- operations/operations-to-research program. This in turn clearly necessitates communication and coordi- nation between research-oriented agencies and operational agencies with end-use requirements so that a robust and adequately funded process exists for transitioning scientifically sound and operationally useful models between the two emphases. Observations are critical for an effective space weather program because they support research and development of models and they drive models in their operational phase. Space weather observations are available from government research and some operational programs as well as from the commercial sector. Currently, each group in isolation develops observational requirements and observing systems to fulfill those requirements. To make more effective use of limited resources, these observations should be nationally coordinated, allowing research groups to provide input and possibly additional payloads to operational or commercial endeavors. Similarly, coordination would allow operational and commercial

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REPORT OF THE PANEL ON SOLAR WIND-MAGNETOSPHERE INTERACTIONS 257 entities to guide and use research observations. It is highly desirable that the National Space Weather Program (NSWP) be augmented to provide a mechanism for coordinating space weather observations, allowing for input to mission definition and payloads from all groups. The SWMI panel encourages the placement and coordination of space-weather-related observing systems on research, commercial, and government platforms to the greatest extent possible. The panel also supports development of mechanisms for real-time data acquisition and distribution from all available platforms. More generally, to make the most of limited resources, an effective space weather program must be accomplished through coordinated activities within the government as well as with the commercial sectors. Currently several commercial and government groups provide various forms of space weather products with some limited coordination, yet many functions overlap. The panel urges that the NSWP be enhanced and clarified in order to delineate the roles for each agency, e.g., NASA, NOAA, NSF, the U.S. Geological Survey, DOD, and DOE, in producing and distributing data and models providing forecasting and space-weather-related products. The panel strongly endorses the undertaking of a high-level study to design a more complete overarching architecture for the NSWP that would define agency roles and coordinate observations. SWMI Imperative: Encourage the creation of a complete architecture for the National Space Weather Program that would coordinate joint research, commercial, and operational space weather observations and define agency roles for producing, distributing, and forecasting space weather products. In addition the SWMI panel encourages all agencies to foster interactions between the research and operational communities and to identify funding for maintaining a healthy research-to-operations and operations-to- research program. Achieving the key science goals of this decadal survey requires identifying the optimum set of operational observations to drive models that will enable specification and prediction of the environment throughout the magnetosphere. This effort may ultimately require an operational “great observatory” of satellites in appropriate orbits for monitoring crucial aspects of the input from and response to solar wind variability. In addition to providing the input necessary for high-fidelity environmental specifications, these measurements would provide routine context information for future targeted science experiments, much as magnetospheric activity indices are used today. Potential elements of such a space weather observa- tory could include a solar wind monitor (including IMF, energetic particle, and potentially coronagraphic measurements); high-altitude synoptic imaging of the aurorae, ring current, plasmasphere, and of the outer magnetospheric boundary; constellation observations of plasma entry and global tail structure; low-altitude, DMSP-like satellites to observe the magnetospheric input into the ionosphere and its response; multiple geosynchronous measurements of plasma, energetic particles, and magnetic field; RBSP-like monitors of the inner magnetospheric radiation environment; and a fine mesh of appropriate ground-based measure- ments. One important near-term investment is to determine, based on past observations of this nature, the optimum set of measurements that are required to drive high-fidelity predictive models of the environment. SWMI Imperative: Implement a program to determine, based on past observations, the optimum set of measurements that are required to drive high-fidelity predictive models of the environment. 9.5.5 Prioritization The SWMI imperatives presented above would all greatly enhance the ability to accomplish the science goals the panel has outlined, thereby providing the foundation needed for addressing the decadal survey’s

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258 SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY TABLE 9.4  Summary of SWMI Prioritized Imperatives Space Rank Imperative Missions DRIVE Weather 1 Enhance the resources dedicated to the Explorer program and broaden the range of cost categories. 2 Complete the strategic missions that are currently in development (MMS and RBSP/ BARREL) as cost-effectively as possible. 3 Initiate the development of a strategic mission, like MEDICI, to determine how the magnetosphere-ionosphere-thermosphere system is coupled and responds to solar and magnetospheric forcing. 4 Ensure strong continued support for extended missions that can still contribute significantly to high-priority science objectives. 5 Enhance and protect support for theory, modeling, and data analysis, including research and analysis programs and mission-specific funding. 6 Ensure continuity of measurements of upstream solar wind. 7 Invest significantly in developing the technologies to enable future high-priority investigations. 8 Ensure strong multiagency support for a broad range of ground-based assets that are a vital part of magnetospheric science. 9 Develop a mechanism within NASA to support rapid development, deployment, and utilization of science payloads on commercial vehicles and other missions of opportunity. NSF and DOD efforts in this regard are also encouraged. 10 Through partnership between NASA’s Heliophysics Division and Planetary Division, ensure that appropriate magnetospheric instrumentation is fielded on missions to other planets. In particular, the SWMI panel’s highest priority in planetary magnetospheres is a mission to orbit Uranus. 11 Partner with other space agencies to implement consensus missions such as a multispacecraft mission to address cross-scale plasma physics. 12 Encourage the creation of a complete architecture for the National Space Weather Program that would coordinate joint research, commercial, and operational space weather observations and define agency roles for producing, distributing, and forecasting space weather products. In addition the SWMI panel encourages all agencies to foster interactions between the research and operational communities and to identify funding for maintaining a healthy research-to-operations and operations-to-research program. 13 Implement a program to determine, based on past observations, the optimum set of measurements that are required to drive high-fidelity predictive models of the environment, and to put in place a plan to ensure that the optimum set of observational capabilities is maintained. 14 Strengthen workforce, education, and public outreach activities. 15 If resources permit, initiate a strategic mission like MISTE to simultaneously measure the inflow of energy to the upper atmosphere and the response of the ionosphere-thermosphere system to this input, in particular the outflow back to the magnetosphere. 16 Create an interagency joint laboratory physics program that addresses issues relevant to space physics.

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REPORT OF THE PANEL ON SOLAR WIND-MAGNETOSPHERE INTERACTIONS 259 key science goals. However, in times of constrained budgets, it may not be possible to enact all of these imperatives, and the panel has, therefore, undertaken a prioritization process to help identify which of these are the most important. The panel’s prioritization—which informed the survey committee but does not carry its imprimatur—is based on the overarching objective to identify initiatives that will most cost- effectively enable the science of the future. Accordingly, the SWMI panel adopted the following criteria: • General considerations for prioritization —Focus on elements that are directly relevant to the enumerated SWMI science goals. —Consider historical evidence about contributions of various elements to scientific progress. —Consider the cost of an imperative versus its likely return. • For mission-related imperatives —How well does the mission address the SWMI science goals? —Is it highly focused or does it address a broader range of the goals? —How feasible is it? —Will it fit within the expected budget? —What is the science return for the cost? —Does it require technology development? — hat is the broader impact (for example, for the science objectives of the other panels and for W the development of a forecasting system for space weather)? • For other capabilities —How central is the capability to the accomplishment of the SWMI science goals? —Is it currently in danger? —Could it make a much bigger contribution with a modest enhancement? Using these criteria, the SWMI panel prioritized its imperatives not only within but also across the three categories discussed above—missions, DRIVE initiative, and space weather—in order to identify the most cost-effective approach to accomplishing its science goals. The full set of prioritized SWMI imperatives, with their mapping to the three categories, is presented in Table 9.4. 9.6  CONNECTIONS TO OTHER DISCIPLINES 9.6.1  Solar and Heliospheric Physics It is clear from the foregoing discussion that the variable solar wind is the dominant driver of magne- tospheric dynamics. To fully understand the ways in which long-term solar variations as well as short-term eruptive events such as solar flares or coronal mass ejections can produce dramatic effects of importance to humans at Earth, solar wind measurements upstream of Earth’s magnetosphere are essential. At present a nearly continuous record of measurements of the solar wind flow velocity, density, and temperature is available from the very early days of the space program up to the present time. Any interruption in the continuity of these measurements would have serious consequences for the ability to study the effects that solar variations have on Earth’s magnetosphere, ionosphere, and atmosphere, and for the ability to forecast significant societal impacts.

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260 SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY 9.6.2  Atmosphere and Ionosphere The past decade has significantly reinforced appreciation of the key influences the ionosphere has on magnetospheric behavior and of the importance of magnetospheric driving for ionospheric behavior. It is thus impossible to understand one without taking into account the two-way coupling with the other. While this decadal survey separates the two regions for ease of discussion, it is clear that they must be understood as a coupled system. During the coming decade one of the most important emphases of space physics research will be to clarify quantitatively the coupling of these elements in order to enable progress toward a predictive understanding of both. 9.6.3  Planetary Science The magnetospheres of other planets display not only certain close similarities, such as the formation of bow shocks and radiation belts, but also many processes that are markedly different, such as the source of charged particles within the radiation belts, which can vary from a mixture of the solar wind and ionosphere at Earth, to lava volcanoes and water geysers on small moons at Jupiter and Saturn. This rich diversity has provided surprising discoveries of the breadth of expression of fundamental physical processes that play important roles in the acceleration of charged particles and the generation of magnetic fields in planetary magnetospheres. Both planetary and magnetospheric understanding is thus enriched by the comparative study of magnetospheres. With orbital missions at many of the solar system’s other planets, comparative magnetospheric studies should increasingly provide insights into the diversity of magnetospheric processes and how they couple to the planets themselves. 9.6.4  Physics and Astrophysics Many of the processes and phenomena that determine magnetospheric behavior act throughout the universe in settings as diverse as laboratory plasmas and supernova shock waves. The ability to observe and diagnose these processes in situ, with no interference from walls, is a powerful tool to enable basic understanding with broad applicability. Examples of such processes include magnetic reconnection, shock acceleration of charged particles, and the physics of rapidly rotating magnetized bodies. Moreover, the study of comparative magnetospheres not only elucidates the behavior of magnetized planets in our solar system, but also points the way to the potential breadth of behavior of magnetized bodies throughout the universe. 9.6.5  Complex Nonlinear System Studies Eruptive solar processes such as solar flares and spontaneously occurring episodic dynamical variations in planetary magnetospheres known as magnetic substorms involve highly nonlinear systems. Similarly, planetary radiation belts evolve through a complicated and nonlinear combination of wave-particle pro- cesses that accelerate particles to very high energies, transport them throughout the system, and cause their loss. Although many of these phenomena are at best poorly understood, their study serves as an important driver for furthering understanding of the mathematical analysis of complex nonlinear systems. Predict- ing the behavior of the vast, coupled, multiscale, and nonlinear Sun-magnetosphere-atmosphere system presents a major challenge to mathematical and computational methods.