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A Science Strategy for Space Physics: Summary A Science Strategy for Space Physics Summary This report by the Committee on Solar and Space Physics and the Committee on Solar-Terrestrial Research recommends the major directions for scientific research in space physics for the coming decade. As a field of science, space physics has passed through the stage of simply looking to see what is out beyond Earth's atmosphere. It has become a "hard" science, focusing on understanding the fundamental interactions between charged particles, electromagnetic fields, and gases in the natural laboratory consisting of the galaxy, the Sun, the heliosphere, and planetary magnetospheres, ionospheres, and upper atmospheres. The motivation for space physics research goes far beyond basic physics and intellectual curiosity, however, because long-term variations in the brightness of the Sun vitally affect the habitability of the Earth, while sudden rearrangements of magnetic fields above the solar surface can have profound effects on the delicate balance of the forces that shape our environment in space and on the human technology that is sensitive to that balance. REPORT MENU The several subfields of space physics share the following objectives: NOTICE MEMBERSHIP SUMMARY To understand the fundamental laws or processes of nature as they PART I apply to space plasmas and rarefied gases both on the microscale and in the PART II larger, complex systems that constitute the domain of space physics; CHAPTER 1 CHAPTER 2 CHAPTER 3 To understand the links between changes in the Sun and the resulting CHAPTER 4 effects at the Earth, with the eventual goal of predicting the significant effects on CHAPTER 5 the terrestrial environment; and PART III APPENDIX To continue the exploration and description of the plasmas and rarefied gases in the solar system. Significant progress has been made in the more than three-decade history of space research. Many space plasma and rarefied gas phenomena have been characterized and are well understood, but many others are still under investigation and new discoveries continue to be made. Space physics asks fundamental questions about how plasmas are energized; about how the energy file:///C|/SSB_old_web/strasum.html (1 of 12) [6/18/2004 2:18:02 PM]

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A Science Strategy for Space Physics: Summary is redistributed with the result that a few particles are taken out of a near-thermal distribution and accelerated to superthermal or very high energies; about the specific roles played by magnetic fields in transferring energy from plasmas to particles and vice versa; about instabilities and interactions between waves and particles in a plasma; about the generation of magnetic fields through convection and rotation; and about the complex physical processes that operate in boundary layers between regions of different types of plasmas and rarefied gases. Some plasma configurations or particle distributions are known to be unstable and to relax spontaneously to a more stable state with the release of free energy, but there are many others for which the instabilities and wave-particle interactions are not yet understood. Determining the physics of such relaxation processes is fundamental to understanding and eventually being able to predict disturbances such as solar eruptions and geomagnetic storms, both of which can have important impacts on a technological society. This strategy identifies five key scientific topics to be addressed in space physics research in the coming decade. For each of these topics, the report presents the scientific background and discusses why the topic is important, describes the current program for research on the topic, and then recommends, in priority order, research activities for the future. As is made clear in the main text, each of these five diverse topics is linked by a number of basic themes. Even though this strategy does not address specific proposals for future programs or missions, consideration is given to the practical aspects of carrying out the recommended investigations. The rationale for the research priorities is driven not only by scientific priority, but also by considerations such as current plans, near-term budget constraints, technological readiness, and balance between large- and small-scale endeavors. The five topics (Box 1), which are not prioritized, and the prioritized recommendations for research in each topic are briefly summarized as follows. Box 1 The Key Topics in Space Physics Research Mechanisms of solar variability q The physics of the solar wind and the heliosphere q The structure and dynamics of magnetospheres and their coupling to q adjacent regions The middle and upper atmospheres and their coupling to regions q above and below Plasma processes that accelerate very energetic particles and control q their propagation THE KEY TOPICS IN SPACE PHYSICS RESEARCH Mechanisms of Solar Variability file:///C|/SSB_old_web/strasum.html (2 of 12) [6/18/2004 2:18:02 PM]

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A Science Strategy for Space Physics: Summary The Sun is a variable star on time scales of milliseconds to centuries or more. These variations occur not only in visible light, but also at radio, ultraviolet, x-ray, and g-ray wavelengths and in the emission of the solar wind and energetic particles. Although solar variability ultimately arises from the interaction of magnetic fields and differential rotation inside the Sun, the Sun's interior dynamics are largely unknown. The new tool of helioseismology is being used to probe the solar interior; it has shown that the Sun's rotation rate does not increase inward as had previously been postulated and thus rules out the "standard" model of the dynamo that generates the solar magnetic field. At least at the solar surface, and perhaps in the interior as well, the magnetic fields are confined to flux tubes—rope-like structures with diameters of about 100 km that are too small to be resolved from Earth. It has been suggested that the twisting and shearing of these flux tubes lead to bursts of high-speed solar wind, called coronal mass ejections, and to solar flares, but the trigger mechanisms for those violent events are not yet known. On longer time scales, complacency about the simplicity of the Sun has been upset by the discovery and documentation in the historical record that the Sun has undergone periods of low activity. The association of the Maunder Minimum period (~1645 to 1715), when few sunspots appeared on the solar surface for roughly 70 years, with the Little Ice Age, when Europe experienced exceptionally cold winters, has potentially serious implications for society should a similar episode occur under current conditions. In addition, confidence in current understanding of the solar interior was upset by the discordantly low flux of solar neutrinos observed in several experiments. It is not yet known whether this disagreement derives from errors in neutrino physics or from errors in understanding of the solar interior. The research priorities for advancing the understanding of solar variability are as follows: Use helioseismology to study the structure and dynamics of the solar surface and interior over a full solar cycle, to obtain information on the interior changes that cause solar cycles. Assure continuity of total and spectral irradiance measurements, supported by spatially resolved spectrophotometry, to investigate correlations between solar magnetic activity and solar output variations and thereby to understand how they are coupled. Measure high-energy radiation and particles from flares and coronal mass ejections with good angular resolution, good spectral resolution, and wide spectral coverage to determine what drives each of those phenomena and how they contribute to the solar output at high energies. Observe surface magnetic fields, velocities, and thermodynamic file:///C|/SSB_old_web/strasum.html (3 of 12) [6/18/2004 2:18:02 PM]

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A Science Strategy for Space Physics: Summary properties with enough spatial resolution (<150 km, with an ultimate goal of <100 km) to study small-scale structures such as flux tubes that may play a decisive role in solar activity and the generation of solar outputs. Make global-perspective measurements of the solar surface magnetic and velocity fields and solar oscillations to measure the three-dimensional structure and long-term evolution of active regions and to detect weak but coherent global oscillations. Measure active regions with angular resolution of ~1 arc sec and temporal resolution of ~10 min for a duration of ~10 days without nighttime gaps to determine the magnetohydrodynamic history of their emergence, development, and decay and the physical scenario behind it. The Physics of the Solar Wind and the Heliosphere Some of the energy transported from the solar interior goes into heating the Sun's outer atmosphere, called the corona, to over a million degrees by processes that are currently the target of intense study. The hot corona in turn becomes the source of the solar wind, but there are still major questions about how this occurs. Further observations and numerical simulations are required to determine the relative importance of magnetic reconnection, explosive jets, tiny active regions called bright points, hydromagnetic waves, and the topology of the magnetic fields in the corona in accelerating the quasi-stationary solar wind. There are additional questions about the acceleration of the nonstationary or transient solar wind arising from explosive events called coronal mass ejections. New observations of the variable elemental composition and ion charge states of the solar-wind plasma are providing valuable clues concerning the acceleration of both types of solar wind. As the solar wind flows out through the solar system, it blows a big bubble, called the heliosphere, in the interstellar medium. Because of its very large scale (~100 AU), the heliosphere provides a unique laboratory for studying plasma processes in relative isolation from boundary effects; from heliospheric studies it is possible to learn much about instabilities in expanding plasmas, the interaction of colliding plasmas, the generation and evolution of plasma waves and magnetohydrodynamic turbulence, and the acceleration and propagation of energetic particles in turbulent fields. Within the decadal time frame considered by this report, it will be possible to measure the latitudinal variations of heliospheric particles and fields over the full range of solar activity and to test theories about the interaction of the solar wind with the interstellar gas and plasma. The following priorities are identified for future research on the solar wind and the heliosphere: Continue to obtain and synthesize the data from the present file:///C|/SSB_old_web/strasum.html (4 of 12) [6/18/2004 2:18:02 PM]

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A Science Strategy for Space Physics: Summary constellation of heliospheric spacecraft and from the interplanetary cruise phases of planetary missions in order to characterize the global and solar-cycle- dependent properties of the heliosphere and its interactions with the interstellar medium. Carry out in situ observations of the solar corona to explore and characterize the region of acceleration of the solar wind and the physical processes responsible for that acceleration. Obtain new types of data required to reveal the mechanisms responsible for the transport of energy, including wave motions (periods of 1 to 10 s), from the solar surface into the chromosphere and corona to understand how these are heated. Carry out stereo imaging of the solar corona to reveal the three- dimensional structure of coronal features without the ambiguity caused by integration along the line of sight. Develop and use techniques for the remote sensing of the coronal magnetic field in order to improve knowledge of the acceleration of the solar wind and of the initiation of coronal mass ejections. Make in situ measurements of the outer heliospheric boundaries and the interstellar medium with instruments specifically designed for those purposes. The Structure and Dynamics of Magnetospheres and Their Coupling to Adjacent Regions Some of the most visually awe-inspiring, yet poorly understood, terrestrial phenomena are a direct consequence of the interaction of the variable Sun and solar wind with the Earth. Auroral displays, usually confined to high latitudes, episodically descend into the temperate zones during periods of extreme solar activity. The aurora is only one manifestation of the complex chain of physical events and connections that link the energy output of the Sun with the Earth's magnetosphere, ionosphere, and atmosphere. As the solar wind reaches the Earth, some of it enters the magnetosphere via several different processes and paths and affects the circulation and dynamics of the plasma within the magnetosphere. The interplanetary magnetic field can become temporarily connected to the geomagnetic field, but the physics of the reconnection process is not yet well understood. Once within the magnetosphere, the energy from the solar wind cascades through the system and some is released catastrophically in events whose trigger mechanisms and extent are not well known. Flows of thermal and energetic plasma, large-scale current systems, magnetic perturbations, and imposed electric fields provide the basic links between the magnetosphere and the ionosphere. The ionosphere file:///C|/SSB_old_web/strasum.html (5 of 12) [6/18/2004 2:18:02 PM]

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A Science Strategy for Space Physics: Summary provides feedback to the magnetosphere in the form of ion outflow, conductivity changes, and dynamo fields. There is a continual reconfiguration of this system as the solar wind and its embedded magnetic field change in response to solar and interplanetary dynamics and energetics. The past and current program, based primarily on in situ measurements, is providing an understanding of the magnetosphere that is strong in terms of local phenomena and a statistical picture of the global structure, but weak in terms of global dynamics. Researchers now know that most transport processes take place within narrow boundary layers connecting regions with very different plasma conditions. The frontier issues for the future center on the global magnetospheric dynamics in response to the solar wind driver, and the physical mechanisms that determine the coupling between regions. Many of the outstanding questions in magnetospheric physics will be addressed by global magnetospheric imaging, a new addition to the techniques available for magnetospheric research. In addition, studies of the magnetospheric environments of other planetary bodies can also yield important physical insights into the mechanisms that drive the dynamical behavior of the Earth's magnetosphere. The following priorities are identified for future progress on this topic: Reap the full scientific potential of the International Solar-Terrestrial Physics program and its coordinated programs to advance understanding of the transport of mass, momentum, and energy throughout the solar wind, and the magnetosphere and ionosphere systems. Simultaneously image the plasma and energetic particle populations in the aurora, plasmasphere, ring current, and inner plasma sheet to study the global structure and large-scale interactions of magnetospheric and ionospheric regions during different levels of solar and geomagnetic activity. Maintain the full complement of particle and field instruments on current and future planetary missions to gain increased understanding of the formation and dynamics of diverse magnetospheres and ionospheres. Further develop and exploit ground-based facilities that image the ionospheric manifestations or "footpoints" of solar wind/magnetosphere coupling processes to complement the magnetospheric imaging initiative aimed at studying the global properties of the magnetosphere. Explore Mercury's magnetosphere to understand the role of an ionosphere in coupling between the solar wind and planetary magnetospheres. Target localized regions that require greater understanding of the small-scale physical processes occurring there with high-resolution, file:///C|/SSB_old_web/strasum.html (6 of 12) [6/18/2004 2:18:02 PM]

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A Science Strategy for Space Physics: Summary multispacecraft measurements that take advantage of new smaller, lighter, more capable instruments and more sophisticated data-compression schemes. The Middle and Upper Atmospheres and Their Coupling to Regions Above and Below A complex interface exists between Earth's space environment and the lower atmosphere or troposphere where weather and climate occur. This interface includes the highly variable middle and upper regions of the atmosphere extending upward from a lower boundary at 10 to 15 km altitude. The middle and upper atmospheres have considerable practical as well as intellectual interest because most ozone resides there and because disturbances in the upper atmosphere and ionosphere caused by solar wind and magnetospheric variations can disrupt technological systems through their effects on satellite drag, communications, and induced ground currents. The middle and upper atmospheres are strongly influenced by inputs of mass, momentum, and energy from both above and below. The absorption of variable solar ultraviolet and x-radiation and of energetic particles not only heats the atmosphere, but also initiates chains of photochemical reactions and ionizes the upper atmosphere to form the ionosphere. Highly variable electric fields and currents originating above and below the upper atmosphere are major sources of energy and momentum to that region. Gravity, planetary, and tidal waves that originate partly from the lower atmosphere grow in amplitude as they propagate upward, where they contribute to the momentum and energy budgets of the middle and upper atmospheres and produce turbulence that influences mixing processes. There are major deficiencies in our knowledge of many of these inputs to the middle and upper atmospheres as well as of the multiple interactions and feedbacks that occur there. The following priorities are identified for future research aimed at understanding this important interface between Earth's lower atmosphere and space: Exploit the exciting new capabilities of UARS,1 FAST, and CEDAR to provide the foundation for future advances in our understanding of the middle and upper atmospheres. Investigate the reaction of the middle and upper atmospheres to upward propagating waves from the lower atmosphere and energy inputs from space so that the sources of important features such as the quasi-biennial and semiannual oscillations and the causes of mesosphere/lower-thermosphere structure and variability can be understood. Study the long-term variations in the middle and upper atmospheres using a combination of consistent long-term satellite and ground-based measurements together with three-dimensional radiative-chemical-dynamical file:///C|/SSB_old_web/strasum.html (7 of 12) [6/18/2004 2:18:02 PM]

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A Science Strategy for Space Physics: Summary modeling to understand natural and anthropogenic changes in these regions. Develop methods to observe the time-dependent electrodynamics operating on microscales to global scales, both in the upper atmosphere- ionosphere-magnetosphere coupling regions so that feedback processes can be characterized, and in the regions above thunderstorms so that the effects of electrified clouds on the "global circuit" and on middle atmosphere chemistry and energetics can be characterized. Take advantage of opportunities to include carefully chosen, appropriate instruments on planetary orbiter missions to make measurements critical to understanding planetary aeronomy and its relation to terrestrial aeronomic processes. Plasma Processes That Accelerate Very Energetic Particles and Control Their Propagation Many of the plasma processes responsible for phenomena within the heliosphere probably also play a role in determining the properties of galactic cosmic rays, which are the only available sample of matter from outside the local solar neighborhood. Mass, charge, and energy spectrometers on existing and planned spacecraft can make in situ measurements of energetic particles throughout the heliosphere to study particle acceleration, fractionation, and transport in a variety of space plasma environments. Theories of acceleration mechanisms in larger-scale galactic structures such as supernova remnants make specific predictions about compositional changes that should take place at the highest energies attainable in those objects, but the theories have not yet been tested observa-tionally. Cosmic rays are confined to the galaxy by turbulent magnetic fields. Measurements of radioactive "clock" nuclei can be used to distinguish diffusive trapping in the galactic halo from the simpler phenomenological models used for many years. Gamma ray measurements can be used to trace the radioactive parent elements of positrons that should be accelerated by the shocks presumed to be the source of the galactic cosmic ray nuclei. Although fluxes of antiprotons and positrons produced in collisions of cosmic rays with gas in the interstellar medium can be calculated with some precision, a full understanding of the sources of antimatter in the cosmic radiation requires a new generation of measurements. Nuclides heavier than nickel are produced by accretion of neutrons either in supernova explosions or during certain other phases of stellar evolution. Knowledge of the abundance of different cosmic ray elements and isotopes will allow the use of nucleosynthesis models to determine quantitatively the fraction of cosmic rays synthesized in each type of source. The abundance of actinide elements can be used as a radioactive clock to determine the time delay between the synthesis of these elements and their acceleration to cosmic ray energies. file:///C|/SSB_old_web/strasum.html (8 of 12) [6/18/2004 2:18:02 PM]

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A Science Strategy for Space Physics: Summary The measurements necessary to address scientific issues concerning particle acceleration and propagation are as follows, in priority order: Complete the observations from the current and planned network of interplanetary spacecraft to study particle acceleration, fractionation, and transport. Extend direct composition measurements to 1015 eV to probe the limits of acceleration and trapping mechanisms. Measure abundances of radioactive isotopes above 1 GeV/nucleon to search for evidence of an extended galactic magnetosphere and wind. Measure the spectra of positrons (10 MeV to 100 GeV) and antiprotons (100 MeV to 100 GeV) to deter-mine where those particles are created and how they are accelerated. Measure isotope abundances for nuclei heavier than nickel and elemental abundances through the actinides to probe the plasma regions where the nuclei are synthesized and to measure the time scales involved. RECOMMENDED RESEARCH EMPHASES The specific programs required to obtain answers to the questions raised under each of the five key topics outlined above are quite different. However, they are united by four common elements or themes that the CSSP and CSTR consider to be the most important research emphases for space physics in the next decade. 1. Complete currently approved programs. The space physics community must reap the benefits of the existing approved programs. A stable program permits the most efficient management and execution of high-priority research. In addition to the obvious scientific return, these ongoing programs provide the basis for developing future research directions. Space physicists will gain the maximum benefit from ongoing and approved missions by: Completing the diverse components of the International Solar- Terrestrial Physics program; Enhancing data analysis and interpretation efforts; file:///C|/SSB_old_web/strasum.html (9 of 12) [6/18/2004 2:18:02 PM]

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A Science Strategy for Space Physics: Summary Streamlining mission operations for all space physics missions; Carrying out extended missions for the uniquely placed Voyager (to the greatest possible heliocentric distance) and Ulysses (through the solar polar passes at solar maximum); Supporting essential observational programs that require long- duration databases; and Enhancing the effectiveness of some of the longer-duration programs by soliciting new ideas and analysis techniques from guest investigators and by ensuring easy access to archived data resulting from the various programs for use in "small science" research tasks. 2. Exploit existing technologies and opportunities to obtain new results in a cost-effective manner. Much technology is already in place to take the next observational steps required to address many of the important questions in space physics outlined here. These steps include: Adapting existing instrumentation to the new generation of smaller spacecraft and more focused space missions; Placing space physics instruments on planetary, Department of Defense, and other spacecraft of opportunity; Utilizing suborbital platforms such as rockets and long-duration balloons for both science objectives and instrument development; and Supporting, where appropriate, activities at unique ground sites such as in the polar cap. 3. Develop the new technology required to advance the frontiers of space physics. In order to achieve several high-priority objectives, or to lower the cost of projects, the limits of technology must be pushed in the following ways: Developing methods to approach the Sun ever more closely to open one of the most exciting new frontiers of space science; file:///C|/SSB_old_web/strasum.html (10 of 12) [6/18/2004 2:18:02 PM]

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A Science Strategy for Space Physics: Summary Producing new spacecraft and instruments based on lightweight structure and miniature electronics; Designing a new generation of instrumentation for remote global imaging of magnetospheric, ionospheric, and solar wind plasmas; Extending capabilities in suborbital techniques for both experimentation and instrument development; Exploiting infrared instrumentation for solar physics; and Devising techniques to explore the region between the altitudes reached by balloons and those reached by spacecraft. 4. Support strongly the theory and modeling activities vital to space physics. Special emphasis should be given to the following topics: Recognizing that synergy between observations, modeling, and theory provides the optimum way of addressing the principal questions in space physics; Making numerical simulations of space physics systems more realistic by extending them to three dimensions, longer time durations, and a greater range of scale sizes, and by incorporating additional physical and chemical processes; Ensuring access to state-of-the-art computational facilities; Exploiting new insights gained from theory, especially the theory of nonlinear processes; and Revisiting earlier efforts to predict solar activity, such as coronal mass ejections and flares, using simulations combined with solar observations. 1A glossary of acronyms is included as the appendix, and the principal programs identified by the acronyms are described in the main text. file:///C|/SSB_old_web/strasum.html (11 of 12) [6/18/2004 2:18:02 PM]