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A Science Strategy for Space Physics: Part I A Science Strategy for Space Physics Part I Introduction to Space Physics Space physics is the study of everything in the solar system above the lower atmospheres of the Earth and other solid bodies. For the purposes of this report, the entire Sun is included in the domain of space physics, together with the upper atmospheres, ionospheres, and magnetospheres of the planets, all the space between the planets, and the cosmic rays that enter the solar system from the galaxy. The purposes of this report are to summarize the state of space physics research in 1995, to identify and explain what the Committee on Solar and Space Physics and the Committee on Solar-Terrestrial Research believe are the most important topics to be addressed during the coming decade, and to suggest and prioritize the types of research efforts that might be most successful in answering the questions raised under each of those topics. REPORT MENU A BRIEF TOUR OF BASIC PHENOMENA NOTICE MEMBERSHIP SUMMARY After 35 years of measurements in space, and even longer periods of studying the PART I Sun, cosmic rays, and the ionosphere from the ground or from balloons, space physicists PART II have a reasonably complete description of the contents of the realm of space physics: CHAPTER 1 CHAPTER 2 CHAPTER 3 The Sun is typical of stars of its mass and age, with its energy generated deep CHAPTER 4 within by the conversion of light elements to heavier elements in a nuclear furnace. The CHAPTER 5 detection of neutrinos produced by this conversion confirmed a fundamental cornerstone of PART III modern astrophysics. The new tool of helioseismology allows us to deduce the distribution APPENDIX of matter, flows, and temperature below the visible surface of the Sun. The vast amount of energy that is constantly working its way from the nuclear furnace to the solar surface causes the gas in the outer layers of the Sun to churn and tumble over itself, which leads to the generation of complex magnetic fields of enormous strength. The combination of the magnetic fields and the churning gas is responsible for the activity seen on the solar surface, of which the best-known features are sunspots and flares. The Sun is constantly changing. No hour goes by without a rise or fall in its output of x-rays; rarely does a day go by without a solar flare; and no decade goes by without a peak and a valley in the Sun's brightness. Although the current decadal change in brightness is between 0.1% and 0.2%, a sample of stars estimated to be similar to the Sun shows variability as large as 0.2% to 0.5%. A comparison of the Sun's present level of file:///C|/SSB_old_web/strapart1.html (1 of 13) [6/18/2004 2:18:20 PM]

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A Science Strategy for Space Physics: Part I activity with a sample of solar-type stars suggests that the Sun's total irradiance may have increased by about 0.25% since the period of low activity in the 17th century known as the Maunder Minimum. It is this solar luminosity (or brightness) that heats the Earth and its atmosphere to habitable levels. Many researchers believe that variations in the luminosity of the Sun were responsible for changes in the climate of the northern temperate zone through the warm and cold centuries recorded by history. Two examples: First, the extremely high solar activity in the 13th century correlates well with abnormal warming that had catastrophic results for the semiarid southwestern United States. Second, during the past century there has been a gradual rise in the level of solar activity. It has been speculated that this rise has been accompanied by an increase in total solar output, and a rise in global temperature. Above its visible surface, the Sun has a tenuous atmosphere, called the corona. Figure 1 is a picture of the corona seen during eclipse, superimposed on a simultaneous x- ray image showing the distribution of hot, dense gases over the surface of the Sun and in the lower corona. The corona is superheated to over a million degrees. It is so hot that it is nearly completely ionized, existing in the fourth state of matter known as a plasma. At some places in the corona the magnetic field is organized into closed loops or bottles that confine the plasma, whereas at other places the field stretches out into space, allowing the outflow of plasma to form the solar wind. At times the magnetic configuration changes suddenly to release additional plasma into the wind. FIGURE 1 Sun and corona: A composite image of the Sun, with north at the top. The x-ray picture of the disk and inner corona was taken with Leon Golub's Normal Incidence X-ray Telescope by a rocket launched from White Sands, New Mexico. The white-light image of the outer corona was obtained simultaneously at 17:30 UT, July 11, 1991, by Serge Koutchmy when the Sun was in eclipse in Hawaii. The black ring visible in some places outside the x-ray solar disk is the moon as seen from Hawaii at the time of the eclipse. (Courtesy of Leon Golub, Smithsonian Astrophysical Observatory.) file:///C|/SSB_old_web/strapart1.html (2 of 13) [6/18/2004 2:18:20 PM]

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A Science Strategy for Space Physics: Part I All the planets are embedded in the solar wind. When the solar wind encounters a planetary magnetic field that extends into space beyond the planet's atmosphere, a magnetosphere is created, because the sunward side of the planetary field is pushed back and confined by the pressure of the wind and, in the down-wind direction, the magnetic field is stretched into a long tail. A magnetosphere absorbs varying amounts of mass, momentum, and energy from the solar wind to drive a variety of phenomena that cause disturbances to a planet's magnetic field, including auroral displays. On Earth, such intense magnetic storms can induce destructive electromotive forces in power grids, disrupt communications, and lead to radiation damage of Earth satellites. When the solar wind encounters the atmosphere of a planetary body that has a weak magnetic field, the interaction may be quite different; at comets, for example, the interaction is dominated by the pickup of ionized cometary material by the solar wind over great distances, whereas at Venus with its higher gravitational field, the major part of the solar wind interaction is with the planet's ionosphere. Above the first 10 km of the Earth's atmosphere lie the layers that constitute the interface between the habitable regions of Earth and space. The ozone and other molecules in those layers absorb most of the harmful components of sunlight and energetic particles precipitating from the magnetosphere. The atmosphere expands and contracts in response to variations in ultraviolet light and x-rays from the Sun, thus dictating the necessary heights of satellite orbits, and occasionally accelerating reentry. The state of the atmosphere above 10 km is sensitive to variations in the input of ultraviolet and x-radiation from the Sun, the flux of energetic particles from the Sun and magnetosphere, and the fluxes from the lower atmosphere of greenhouse gases (principally carbon dioxide and methane) as well as gases that affect ozone chemistry (such as the chlorofluorocarbons and methyl bromide). The solar wind continues well past the outermost planet and is expected to come to a stop only when it reaches a pressure balance with the interstellar medium; the region of space occupied by the solar wind is called the heliosphere. Cosmic rays, which are nuclear particles accelerated to enormous energies elsewhere in the galaxy, push their way into the heliosphere in opposition to the outwardly expanding solar wind. The sources of cosmic rays are apparently distributed throughout the disk of the Milky Way galaxy, and the particles fill the galaxy, perhaps driving a galactic wind in a manner analogous to that by which the Sun drives the solar wind. The study of their energy spectrum and composition permits inferences not only about the origin and evolution of the elements, but also about the structure and evolution of the galaxy. Space physicists now have some idea of the average properties-such as the density, temperature, flow patterns and speeds, and magnetic fields-at most locations within the heliosphere. The exceptions include the unexplored magnetosphere of Pluto (if it has one), some regions of other planetary magnetospheres, the innermost heliosphere, high solar latitudes under conditions of maximum solar activity, the distant outer region of the heliosphere, and the interstellar medium beyond the heliosphere. This brief tour highlights some of the basic phenomena of space physics. Although there is great diversity in the settings and the details, only a few basic processes underlie much of the physics. Mass motions in ionized media lead to the creation of magnetic flux in natural dynamos, such as the solar interior and the Earth's upper atmosphere. Further mass motion and the interaction of charged particles with the field and with neutral gases lead to file:///C|/SSB_old_web/strapart1.html (3 of 13) [6/18/2004 2:18:20 PM]

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A Science Strategy for Space Physics: Part I the dissipation of the magnetic energy through magnetic reconnection and other processes. Some of the energy is used to accelerate a small fraction of the matter to extraordinary energies. For the most part, the emphasis in space physics research has shifted from exploration to trying to understand why the properties are as we observe them to be. Instead of asking only, What's out there?, space physicists are increasingly asking, How does it work? and How will it evolve? This is especially true of the dynamic events, from solar eruptions to terrestrial magnetic storms to environmental changes. There is much still to be learned: Why is the output of neutrinos by the solar furnace so much less than the theoretically predicted value? How does the Sun generate the strong magnetic fields observed at its surface? What processes cause the solar variability observed on time scales from seconds to centuries? How is the solar corona heated, and how is the solar wind accelerated to the high speeds observed? How do changes in the solar wind affect the various planetary magnetospheres? How are processes in the Earth's magnetosphere, ionosphere, and different levels of the atmosphere coupled together? What are the processes responsible for the observed dynamical and chemical variability and for the energy budget of the different layers of the upper atmosphere? How do magnetic fields and plasma particles interact on the large scale of the heliosphere? What is the origin of cosmic rays, and how do they interact on galactic scales? THE OBJECTIVES OF SPACE PHYSICS The scientific objectives of space physics research are as follows: To understand the fundamental laws or processes of nature as they apply to space plasmas and rarefied gases both on the microscale and in the larger, complex systems that constitute the domain of space physics; To understand the links between changes in the Sun and the resulting effects at the Earth, with the eventual goal of predicting the significant effects on the terrestrial environment; and To continue the exploration and description of the plasmas and rarefied gases in the solar system. Fundamental Processes Plasmas are the fourth state of matter. A plasma behaves differently than does a solid, liquid, or gas because each of its charged particles reacts to the electric and magnetic fields generated by the locations and motions of all the other charged particles within a very large volume; in turn, the resulting changes in each particle's location or motion cause changes in the fields. After more than three decades of research, many plasma phenomena are well understood. One example is the propagation of a host of different types of waves file:///C|/SSB_old_web/strapart1.html (4 of 13) [6/18/2004 2:18:20 PM]

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A Science Strategy for Space Physics: Part I (e.g., electromagnetic, electrostatic, fast- and slow-mode magnetosonic, Alfvén, and various hybrid waves), at least at small amplitudes where nonlinear effects are not important. But many other plasma processes require further investigation, both observationally and theoretically. Some of the major processes of current concern in space physics are the following: Bulk energization or acceleration of plasmas; Acceleration of a few particles out of a near-thermal distribution to superthermal or very high energies; Magnetic reconnection in which the topology of the magnetic field is changed, sometimes very rapidly, and magnetic energy is converted to kinetic energy of the particles in the plasma; Instabilities and interactions between waves and particles in a plasma. Some plasma configurations or particle distributions are known to be unstable and to relax spontaneously to a state of lower free energy, but there are many others for which the details of the instabilities and wave-particle interactions are not yet understood; Wave-wave interactions, both in plasmas and neutral gases; Magnetohydrodynamic turbulence and turbulent diffusion; and Dynamo generation of magnetic fields in a star with a convective envelope (such as the Sun's). It is seldom possible to study any single physical process in isolation from myriad other simultaneously occurring processes. In space, it is nature rather than the investigator that determines the observational parameters and the boundary conditions. Thus we are constrained to observe processes as they are found within large-scale systems, such as the Earth's magnetosphere, ionosphere, or mesosphere, the solar corona, or the heliosphere, to name only a few examples. But those large-scale systems are also of intrinsic interest in themselves. Consider the magnetosphere as one example. It continuously intersects flows of mass, momentum, and energy in the solar wind, absorbs a varying fraction of those flows, rearranges itself as it temporarily stores this mass and energy, and then sporadically sheds this input to maintain a steady state when averaged over the long term. Researchers still have much to learn about how the magnetosphere does all that using basic physical processes such as magnetic-field reconnection, plasma instabilities, and a host of others. Application to the Terrestrial Environment The fundamental physics of rarefied gases and plasmas has practical application to the problem of understanding the impact of solar processes on the terrestrial environment. A recent NRC study1 chaired by Judith Lean reached the following conclusions concerning solar influences on global change: file:///C|/SSB_old_web/strapart1.html (5 of 13) [6/18/2004 2:18:20 PM]

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A Science Strategy for Space Physics: Part I Solar variations directly modify global surface temperature. Solar variations modify ozone and the structure of the middle atmosphere. The effects of solar variability in the Earth's upper atmosphere may possibly couple to the biosphere. It is not known whether or not other types of variability in the Earth's near-space environment couple to the biosphere. Improved knowledge of the Sun is required to understand and predict the influences of solar variability on global change. The CSSP-CSTR endorses the prioritized recommendations for monitoring and scientific research developed by the Lean Committee; a summary of those recommendations is reproduced in Box 2. Box 2 Recommendations of the Report Solar Influences on Global Change The highest priority and most urgent activity for determining solar influences on global change is to: 1. Monitor the total and spectral solar irradiance from an uninterrupted, overlapping series of spacecraft radiometers employing in-flight sensitivity tracking. So that the long term value of present solar monitoring is not lost, adequate temporal overlap to permit cross-calibration with future observations is critical. This goal must be achieved in an era of decreasing access to space. In addition, the following activities will be needed to properly monitor, understand, and predict solar influences on global change. Pursuit of recommendations 2 to 6 is essential to the interdisciplinary research effort needed to provide an adequate scientific basis for global change policymaking. The actions of recommendations 7 to 12 are essential to ensure a complete understanding of all potential coupling mechanisms. 2. Conduct exploratory modeling and observational studies to understand climate sensitivity to solar forcing. 3. Understand and characterize, through analysis of solar images and surrogates, the sources of solar spectral (and hence total) irradiance variability. file:///C|/SSB_old_web/strapart1.html (6 of 13) [6/18/2004 2:18:20 PM]

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A Science Strategy for Space Physics: Part I 4. Monitor, without interruption, the cycles exhibited by Sun-like stars and understand the implications of these observations for long term solar variability. 5. Monitor globally, over many solar cycles the middle atmosphere's structure, dynamics, and composition, especially ozone and temperature. 6. Understand the radiative, chemical, and dynamical pathways that couple the middle atmosphere to the biosphere, as well as the middle atmosphere processes that affect these pathways. 7. Monitor continuously, with improved accuracy and long term precision, the ultraviolet radiation reaching the Earth's surface. 8. Understand convection, turbulence, oscillations, and magnetic field evolution in the solar plasma so as to develop techniques for assessing solar activity levels in the past and to predict them in the future. 9. Monitor continuously the energetic particle inputs to the Earth's atmosphere. 10. Monitor the solar extreme ultraviolet spectral irradiance (at wavelengths less than 120 nm) for sufficiently long periods to fully assess the long term variations. 11. Monitor globally over long periods the basic structure of the lower thermosphere and upper mesosphere so as to properly define the present structure and its response to solar forcing. 12. Conduct observational and modeling studies to understand the chemical, dynamical, radiative and electrical coupling of the upper atmosphere to the middle and lower atmospheres. Note: Reprinted from pp. 10-12 of Board on Global Change, Solar Influences on Global Change, National Academy Press, Washington, D.C., 1994. Disturbances propagating through the Earth's space environment are known to affect a number of modern technological systems. A schematic illustration of some of these effects, both in space and on the surface of the Earth, is shown in Figure 2. For example, the x-rays emitted during solar flares travel to Earth and are absorbed in the upper atmosphere. This leads to changes in the electron density and the currents in the lower ionosphere, which cause radio waves to propagate in different and often unexpected directions. Signals from communication satellites are similarly affected by severe geomagnetic activity. The variability of the Sun's radiation produces variable heating of the upper atmosphere, which results in increased satellite drag, and perhaps premature reentry. file:///C|/SSB_old_web/strapart1.html (7 of 13) [6/18/2004 2:18:20 PM]

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A Science Strategy for Space Physics: Part I FIGURE 2 Some effects of solar-terrestrial disturbances. (Courtesy of the Space Environment Laboratory, NOAA.) Another impact on human systems comes in the form of energetic particles. The particle radiation level in the Earth's space environment can become hazardous for astronauts and electronic systems on satellites, as illustrated in Figure 2. The increased radiation due to solar energetic particle events and high levels of geomagnetic activity cause surface charging and deep dielectric charging on satellites, which can result in electronic faults and permanent damage to electrical systems. In the most extreme cases, satellites have become useless or inoperable due to these effects. High levels of geomagnetic activity are capable of disabling entire utility systems. Large-scale blackouts can have serious economic impacts even if power is restored in a few hours. The economic impact of a four-hour major blackout in France was estimated to be $1 billion, while a somewhat longer major blackout of the Northeast section of the United States could result in a $3 billion to $6 billion loss.2 These problems are a consequence of geomagnetically induced currents that flow in the neutrals of grounded power transformers and lead to saturation of the core, with resulting transformer heating and failure. While there are numerous other examples, a clear trend has emerged: as technology advances, an increasing number of systems are affected by the highly variable space weather.3 Exploration There are several important regions of the solar system where the properties of plasmas and rarefied gases have not yet been determined. Surprises almost certainly are in store for us when we obtain data from the first spacecraft sent to many of those regions. A recent example is the discovery of intense plasma waves, very high fluxes of energetic particles, and large fluxes of negative ions detected at comets. The lessons learned about fundamental physical processes, such as ion pickup, from the comet flybys of 1985 to 1992 have broad applicability in other large-scale systems, such as the outer heliosphere. file:///C|/SSB_old_web/strapart1.html (8 of 13) [6/18/2004 2:18:20 PM]

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A Science Strategy for Space Physics: Part I Examples of regions about which space physicists currently know very little and that thus still need to be explored include the magnetic field in the local interstellar medium or the nature of the particle distributions or the wave field in the region close to the Sun where the solar wind is accelerated. THE ROLE OF NEW TECHNOLOGY Advances in each of the three areas discussed above are enabled by the development and application of relevant new technology. Some progress can be made by sending old instruments to new places. However, many critical measurements described in this report require greater sensitivity or better resolution (be it spatial, temporal, energy, angular distribution, and so on) than can be achieved with currently available hardware. Advanced computers and software are required to generate models that can account for and visualize all the relevant spatial and temporal scales involved in some of the more challenging problems. Several of the high-priority objectives of space physics require a new generation of very lightweight, low-power instruments. BACKGROUND AND SCOPE OF THIS STUDY For many years, the Space Studies Board (SSB) and its predecessor, the Space Science Board, have provided scientific advice to NASA via the mechanism of discipline- specific strategy reports. Such reports are intended to assist NASA in developing the best possible scientific research programs for the future. In 1980, the Committee on Solar and Space Physics (CSSP) produced its first strategy report, Solar-System Space Physics in the 1980's: A Research Strategy,4 which is often referred to as the Kennel report. That report identified the scientific objectives for solar and space physics and described the program, experiments, and instruments required to continue progress on a broad front toward achieving those objectives. In 1985, the CSSP published An Implementation Plan for Priorities in Solar-System Space Physics5 (the Krimigis report), which, starting with the science objectives given in the Kennel report, developed a plan for addressing those objectives. The Committee on Solar-Terrestrial Research (CSTR) of the Board on Atmospheric Sciences and Climate also published reports during the 1980s-Solar-Terrestrial Research for the 1980's,6 and National Solar-Terrestrial Research Program7-that recommended science programs in solar and space physics for all federal agencies involved in such research, including ground-based as well as space-based activities. In addition, during the decade of the 1980s a number of other reports were produced that addressed specific issues or areas of solar and space physics, including upper atmospheric research, the Middle Atmosphere program, international cooperation, the Explorer program, data management, ground-based solar physics, and long-term solar-terrestrial observations. In 1992, the CSSP and CSTR, operating together as a single, federated committee, completed an assessment of the status of the field and a review of the federal agencies' responses to past recommendations (Assessment of Programs in Solar and Space Physics- 1991).8 The CSSP-CSTR followed this assessment report with the report A Space Physics Paradox: Why Has Increased Funding Been Accompanied by Decreased Effectiveness in file:///C|/SSB_old_web/strapart1.html (9 of 13) [6/18/2004 2:18:20 PM]

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A Science Strategy for Space Physics: Part I the Conduct of Space Physics Research?.9 The concluding recommendation of the "Paradox report" was that, in anticipation of an era of limited resources, the space physics community could ensure the most effective use of funding for space physics research by establishing priorities across the full spectrum of its scientific interests, encompassing both large-scale and small-scale activities. Other motivations for updating the science strategy for solar and space physics are the following: There have been both scientific and technological advances in the field in the 15 years since the publication of the Kennel report and the 10 years since the publication of the Krimigis report. Many of NASA's and NSF's plans for future programs address new scientific issues that were not considered in those earlier reports. Changes in the NASA organization have resulted in upper atmosphere research and cosmic-ray physics being administered as part of space physics; a broader scientific rationale and view are therefore required for the assessment and direction of NASA's space physics program. Changes in the roles of the shuttle and the space station in NASA's program, coupled with changes in the fiscal environment in which the relevant federal agencies operate, have made parts of the previous strategies obsolete. As a result of the tight budget situation, the scientific community is under increasing pressure from the funding agencies and from Congress to set priorities for future research. An observation made by Rep. George E. Brown, Jr., former chairman of the House Committee on Science, Space, and Technology, can serve to emphasize this last point: As the space program has matured and we have come to understand the full potential for the space sciences, the necessity to set priorities has become essential. In a constrained budget environment priority-setting is crucial. . . . If scientists fail to [set priorities], science funding may be increasingly decided by "political pork" awarded to localities based on political rather than scientific goals. For these many reasons, the SSB asked the CSSP-CSTR to undertake the development of a new science strategy to assist science planning efforts for space physics. This report is the CSSP-CSTR response to that request. The report covers the entire spectrum of subdisciplines that make up the discipline of space physics. It encompasses both big science and little science endeavors and the full spectrum of research methods, including observation (both from space and from the ground), theory, and numerical simulation. Although the emphasis is on the U.S. national program, the science objectives transcend national boundaries, and the study takes into account relevant foreign efforts. While reflecting full cognizance of current and approved programs, in addressing areas of future scientific emphasis the report does not assess any agency's specific implementation plans for future missions or programs. It also does not consider institutional issues (e.g., universities vis à vis NASA or NSF centers) or other issues associated with the science infrastructure. file:///C|/SSB_old_web/strapart1.html (10 of 13) [6/18/2004 2:18:20 PM]

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A Science Strategy for Space Physics: Part I The subject matter of the present report overlaps and complements that found in several recent National Research Council science strategy reports. First, the CSSP and CSTR acknowledge and have taken into consideration the work of the Solar Astronomy Panel of the NRC Astronomy and Astrophysics Survey Committee10,11 and of the recently completed study of the impact of solar processes on the terrestrial environment.12 The committees' consideration of cosmic-ray physics is more focused on the plasma physical aspects of that field than is the concurrent study by a panel of the Board on Physics and Astronomy that emphasizes the astrophysical context.13 Perhaps the greatest overlap with the present study is evident in one recently completed by the Committee on Lunar and Planetary Exploration (COMPLEX),14 which reviewed the status of magnetospheric and atmospheric research for each of the planets as well as the small bodies of the solar system. Although the CSSP-CSTR study is more terrestrially oriented, the committees recognize that comparison of the similarities and differences between the phenomena observed in different settings can yield greater additional insight into the fundamental atmospheric and plasma processes and can serve as a cross-check on the interpretation of the observations of any single body. The CSSP and CSTR concur with and support the relevant findings and recommendations of those other studies and have considered their conclusions in the process of determining the strategy for space physics. Finally, the committees note that although the report often speaks in terms of research to be carried out during the coming decade, it is not possible to provide a realistic, detailed schedule, especially in view of the present volatility of science budgets in all agencies. The CSSP and CSTR can only urge that the program outlined here be implemented as rapidly and as vigorously as possible. NOTES 1. Board on Global Change, National Research Council, Solar Influences on Global Change, National Academy Press, Washington, D.C., 1994. 2. Barnes, P.R., and J.W. Van Dyke, "Potential Economic Costs from Geomagnetic Storms," IEEE Spectrum, March, 1990. 3. The CSTR-CSSP is currently developing a research briefing report on the topic of these "space weather" causes and effects. 4. Space Science Board, National Research Council, Solar-System Space Physics in the 1980's: A Research Strategy, National Academy Press, Washington, D.C., 1980. 5. Space Science Board, National Research Council, An Implementation Plan for Priorities in Solar-System Space Physics, National Academy Press, Washington, D.C., 1985. 6. Geophysics Research Board, National Research Council, Solar-Terrestrial Research for the 1980's (Herbert Friedman and Devrie Intriligator, co-chairs), National Academy Press, Washington, D.C., 1981. 7. Board on Atmospheric Sciences and Climate, National Research Council, file:///C|/SSB_old_web/strapart1.html (11 of 13) [6/18/2004 2:18:20 PM]

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A Science Strategy for Space Physics: Part I National Solar-Terrestrial Research Program (Devrie Intriligator, chair), National Academy Press, Washington, D.C., 1984. 8. Space Studies Board, National Research Council, Assessment of Programs in Solar and Space Physics-1991, National Academy Press, Washington, D.C., 1991. 9. Board on Atmospheric Sciences and Climate and the Space Studies Board, National Research Council, A Space Physics Paradox: Why Has Increased Funding Been Accompanied by Decreased Effectiveness in the Conduct of Space Physics Research?, National Academy Press, Washington, D.C., 1994. 10. Board on Physics and Astronomy, National Research Council, The Decade of Discovery in Astronomy and Astrophysics, National Academy Press, Washington, D.C., 1991. 11. Board on Physics and Astronomy, National Research Council, Working Papers: Astronomy and Astrophysics Panel Reports, National Academy Press, Washington, D.C., 1991. 12. Board on Global Change, National Research Council, Solar Influences on Global Change, National Academy Press, Washington, D.C., 1994. 13. Board on Physics and Astronomy, National Research Council, Opportunities in Cosmic-Ray Physics and Astrophysics, National Academy Press, Washington, D.C., 1995. 14. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995-2010, National Academy of Sciences, Washington, D.C., 1994. file:///C|/SSB_old_web/strapart1.html (12 of 13) [6/18/2004 2:18:20 PM]