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Introduction To understand the relationship between natural events on Earth and changes in the Sun has been one of marks enduring intellectual quests. The scientific discipline we now know as so- lar system space physics is the modern culmination of efforts to comprehend the relationships among a broad range of naturally occurring physical ejects including solar phenomena, terrestrial magnetism, and the aurora. Understanding the solutions to these basic physics problems requires the study of ionized gases (plas- mas), magnetohydrodynamics, and particle physics. Space physics ~ an identifiable discipline began with the launch of the first earth satellites ~ the late l950s and the dis- covery in 1958 of the Van Allen radiation belts. The phenomena associated with this field of study are among the earliest recorded observations in many parts of the world. The ancient Greeks were puzzled by the refires in the upper atmosphere that we now call the aurora; there are several possible references to the aurora in ancient Chinese writings before 2000 B.C.; there are also passages in the first chapter of Ezekiel with vivid descriptions of what we now recognize as auroral formations. The observation of sunspots by Galileo in 1610 led to the eighteenth-century discovery of the 11-year solar sunspot cycle and the recognition that there was a connection between sunspot 1

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2 variability and auroral activity. The large reduction of sunspots during the second half of the seventeenth century during a period of unusually coo} weather in Europe suggests a tantalizing connection between some aspects of solar activity and climate. This possible link between solar activity and terrestrial phe- nomena could not be studied in detail until this century. We now know that, in addition to the atmosphere that surrounds us, there exists a region, at higher altitudes, consisting of an electrically conducting plasma permeated by the Earth's magnetic field. It is called the "magnetospheres because its structure and many of its processes are controlled by the magnetic field. Since the early years of the space program, we have learned that the Sun has its own magnetosphere consisting of a hot (million degree KeIvin) magnetized plasma wind (the solar wind) that extends beyond the orbits of the planets and fills interplanetary space, forming a dis- tinct cavity in the nearby interstellar medium the "heliosphere." Using knowledge gained over the past 25 years, we can now begin to identify some of the physical mechanisms linking the Sun to our near-Earth environment. For example, motions in the convective layers of the Sun are believed to generate the solar magnetic field and solar wind variations; these in turn affect the Earth's magnetosphere and regulate the amount of plasma energy incident on the Earth's polar caps. Associated magnetospheric activity drives strong winds in the upper atmosphere and may in- fluence the dynamical and chemical composition of the mesosphere and stratosphere as well. The upper atmosphere, in turn, is the major source of heavy ions in the magnetosphere. Further, current research suggests that small percentage changes (about 0.5 per- cent) in the total energy output of the Sun (the solar "constant") may influence short-term terrestrial climate. These and other speculative suggestions should be addressed as part of a compre- hensive research program in solar-terrestrial physics because of their potential importance for the Earth. Indeed, the Earth and its space environment contain coupled phenomena and need to be studied as a system from the Sun and its plasma environment to the Earth's magnetosphere, atmosphere, oceans, and biota. Discoveries in solar and space physics over the past 25 years have inspirer} a number of developments in theoretical plasma physics. Concepts in charged particle transport theory, developed to describe the behavior of energetic particles in the solar wind and magnetosphere, are routinely used in studying extragalactic

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3 radio sources and laboratory plasmas. Magnetic field reconnection Convolving the explosive conversion of electromagnetic energy into particle energy), collisioniess shock waves, electrostatic shocks, and hydromagnetic turbulence are also among the fundamental plasma phenomena first studied and elucidated in analyses of solar and space plasmas. Subsequently, these and other concepts have found application to related branches of plasma physics, such as nuclear fusion. The development of space plasma physics since the 1960s has influenced nuclear fusion research. Pitch-angle scattering and magnetic re- connection are now took of laboratory plasma theory, while ideas developed in fusion work have influenced space plasma science in unportant ways. Thus, the language of plasma physics links two very important scientific endeavors: the search for a limit- less supply of clean energy through thermonuclear fusion and the exploration and understanding of our solar system environment, most of which is in the plasma state. New concepts developed in studies of solar and space plasmas find important applications to astrophysical problems as well as to laboratory plasmas. For example, the structure of collision- less shock waves can be resolved only by spacecraft instruments. Such shocks are invoked In some current models of star formation. Furthermore, the study of propagating interplanetary shocks has contributed to understanding and modeling of acceleration of cos- rnic rays by shocks. Particle acceleration via direct electric fields, observed in the Earth's magnetosphere, has been invoked in accel- eration models of pulsar magnetospheres. The subject of cosmic- ray transport owes much to detailed in situ studies of the solar wind. Some stellar winds are thought to be associated with stars that, like the Sun, have convective outer layers, while winds of more massive stars are driven by racliation pressure. Explanations of physical phenomena in astrophysical objects that will remain forever inaccessible to direct observation rest heavily on insights obtained through studies of solar system plasmas accessible to in situ observations. Even though space plasma physics AS a mature subject, new oh servations continue to reveal facets of the physics not recognized previously. For example, observations of "spokes in Saturn's rings seemed to highlight the importance of electromagnetic forces On charged dust particles. Similarly, the interaction of dust and

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4 plasma in comets is thought to be a central element in understand- ing the formation of comet ion tails. Such observations have given rise to the study of "gravito-electrodynamics~ in dusty plasmas, which in turn has important applications to the understanding of the formation and evolution of the solar system, as pointed out by Alfven some years ago. The unclerstand~ng of the near-Earth space environment is not only a basic research enterprise; it also has extremely impor- tant practical aspects. Space is being used increasingly for many different scientific, commercial, and national security purposes. Well-known examples include communications and surveillance satellites and such scientific platforms as the Space Telescope and the Space Station. These space vehicles must function contin- uously in the near-Earth environment, subject to the dynamic variations of the heliosphere, the magnetosphere, and the upper atmosphere. It is well established that many spacecraft systems and subsystems exhibit anomalies, or even failures, under the influ- ence of magnetospheric substorms, geomagnetic storms, and solar flares. Processes such as spacecraft charging and "single-event upsets" (owing to highly ionizing energetic particles) in processor memories make the day-to-day operation of space systems diffi- cult. Finally, these aspects of the near-Earth environment become particularly important in view of the planned long-term presence of man in space. The complement of programs outlined in this report will allow us to mode! the global geospace environment and will thus allow us to develop a global predictive capability. This, in turn, should permit substar~tial improvements in our abilities to operate all space-based systems in the near-Earth region. We have advanced well beyond the exploratory stages in solar and space physics, with some notable exceptions the solar inte- rior, the environment near the Sun where the solar wind is accel- erated, the atmospheres of some of the planets, and the boundary of the heliosphere. The phenomenological approach appropriate to a young science still in its discovery phase ho progressed to a more mature approach where focused and quantitative investiga- tions are made, interactive regimes are studied, and theory and modeling play a central role in advancing understanding. The future solar and space physics program will require tools and techniques substantially different from those of the past. Con- tinued progress will require development of complex, multifaceted,

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5 experimental and observational projects that will be technologi- cally challenging. The task group believes that the anticipated scientific contributions fully justify the proposed undertakings. The purpose of this report is to develop an overall program of space research that will address the most significant topics in this discipline, that will clearly define the priority of investigations, and that will be affordable by NASA. Several other National Research Council reports that are re- lated to this document have appeared in recent years. The Colgate report (Space Plasma Physics: The Study of Solar-System Plas- mas, 1978) reviewed the status of the field and concluded that "space plasma physics is intrinsically an important branch of sci- ence." The Kennel report (Solar-System Space Physics in the 1980~: A Research Strategy, 1980) laid out the scientific goad and objectives for the field. Other reports (Solar-Terrestrial Research in the 1980s, 1981; National Solar-Terrestrial Research Program, 1984) integrated the ground-based segment of the field and stated priorities for its unplementation. The Physics of the Sun (1985) reviewed the scientific content of solar physics and described future research directions. A Strategy for the Explorer Program for Solar and Space Physics (1984) emphasized the need for a revitalized Explorer program for solar and space physics and outlined several specific examples of scientific investigations. An Implementation Plan for Priorities in Solar-System Space Physics (1985) updated the scientific goals and objectives of solar and space physics re- search from the Kennel report and developed the prioritized im- plementation plan for NASA that would accomplish these aims. Chapter 2 of the task group's report reviews those scientific objectives, and Chapter 3 describes the status of solar and space physics expected in 1995, assuming a number of space programs proceed according to present plans. In Chapter 4, a variety of new programs intended for imple- mentation after 1995 are identified that will employ new techniques for the investigation of outstanding scientific questions, will ad- dress new questions that arise as natural extensions of previous studies, and will allow the pursuit of new topics that we cannot address at present. This chapter also describes the developments in technology that will be required for the era after 1995; Chapter 5 summarizes the technology needs. This report also contains a number of appendixesreports (or excerpts of reports) of workshops that were conducted by NASA in

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6 support of this study. The workshops brought together scientists from diverse backgrounds to explore new ideas and technologies for space science research. Those efforts were an important part of the study they were largely responsible for some of the new initiatives proposed in Chapter 4.