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4 Solar System Space Physics BACKGROUND Solar system space physics is concerned primarily with the sources and behavior of ionized gas (plasma) in the solar system. Plasma Is sometunes called the fourth state of matter. Although solids, liquids, and electrically neutral gases are more familiar in everyday life, ionized gas is the most abundant state of matter in the universe. Plasma processes are essential to the physics of the Sun, as wed as many other phenomena of the solar system and beyond. The goals of solar system space physics are to understand: . The physics of the Sun: its extended ionized atmosphere (the interplanetary medium), the magnetospheres, ionospheres, and upper atmospheres of the Earth, other planets, and comets, and the propagation of cosmic rays in the interplanetary medium. . The processes that link solar variations to terrestrial phe- nomena. Such processes reveal basic physical mechanisms and influence many circumstances of human endeavor. Study of the Sun, our nearest star, provides a firm basis for un- derstanding stellar processes and astrophysical plasmas. Sunlight in the visible and near-ultraviolet portions of the electromagnetic 27

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28 spectrum sustains nearly all life on Earth. However, there are many subtle and less weD understood influences of the Sun's em~s- sions in other portions of the electromagnetic spectrum and ~ the form of high-speed ionized gases (the solar wind) and energetic particles. The interactions of these radiations with the neutral atmospheres and plasma environments of Earth and other planets cause a variety of physical phenomena. Some of these phenom- ena affect human activities, but all of them are of great scientific interest. The Bight of sophisticated instruments, first by high-altitude rockets beg~nn~g ~ 1946, and more recently by earth satellites and interplanetary spacecraft, has revolutionized research in solar and space physics. Indeed, this field of research has been one of the most successful of Al of the space sciences. Observations above Earth's atmosphere have led to the discovery of solar ultraviolet, x- ray, and gamma-ray ern~ssions, Al of which arise from nonthermal processes. These observations provide the first undistorted view of the small-scale plasma structures that control such nonthermal processes. At altitudes far above the atmosphere, there is a region con- taining electrically conducting plasma and a huge population of high-energy particles trapped in Ea~th's external magnetic field. This region is called the magnetosphere. Recent investigations re- veal that Jupiter, Saturn, and Uranus have magnetospheres whose dimensions are on the order of millions of kilometers. While the magnetospheres of the planets in the solar system exhibit certain similarities, each is distinctive in detail. In addition, there are plasma phenomena associated with each of the inner planets- Mercury, Versus, Earth, and Marsand with comets. A common element in Al of these physical systems Is the "solar wind, the flowing solar plasma that permeates the solar system. The re- gion around the Sun where this solar wind occurs Is called the "heliosphere.~ The boundary between the heliosphere aIld the interstellar medium has not yet been observed, but Is estunated to be at a distance of 50 to 100 AU from the Sun, beyond the orbits of Al known planets. The observation of this boundary between the heliosphere and the local interstellar medium is one of the central objectives of contemporary space physics. Solar shy tem space physics research encompasses all of the plasma physical phenomena within the heliosphere.

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29 TEE SUN, SOLAR PROCESSES, AND VARIABILITY The Recovery of sunspots by Galileo in 1610 led to recogni- tion, in the eighteenth century, of the 11-year cycle of sunspot number and other indices of solar activity. We know now that such activities have tune scales ranging from a few seconds to at least 22 years. Giant solar Dare eruptions occur at frequent intervals and release large amounts of energy into interplanetary space. This energy includes radiation that spans ahnost the en- tire electromagnetic spectrum and energetic particles sufficiently intense to kill exposed living matter ~ space. Reliable forecast- ing of such activity is one of the challenges of solar physics. The large-scale structures observed on the Sun are thought to be the macroscopic manifestations of small-scale plasma processes asso- ciated with turbulent magnetic fields. The Sun's brilliance and large apparent size offer unique opportunities for observing its un- derlying physical processes in great detail. Even the shape of the solar globe is not constant. It oscillates with periodicities from rn~Hiseconds to hours. There is an intonate connection between variations in the visible surface of the Sun (the photosphere) and processes deep within its interior. Recent work in studying these oscillations is making it possible to use a new technique, called ~helioseismology," to penetrate the opaque brilliance of the solar surface. In mum the same way as geophysicists study seismic waves to learn about conditions withm Earth, solar physicists are exploiting natural oscillations of the Sun to probe its interior. TEE SUN-EA}ITH SYSTEM, THE MAGNETOSPE1DRE, AND THE AURORA The m~Dion-degree outer atmosphere of the Sun expands in all directions at supersonic speed (about 400 km/s) and envelops Earth and all known planets. Earth's magnetic field deflects this flow of hot plasma from impinging directly onto its atmosphere. In the process, the magnetic field is compressed on the dayside and extended on the nightside into a long plasma magnetic tail like the visible tail of a comet. The solar wind power imps ng on the magnetosphere is some 50 trillion watts, about 100 times the electrical generating capacity of the United States. A fraction of this energy, in the form of hot plasma, eventually penetrates the magnetic field and circulates through the magnetospheric system. -

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30 Some of this energy (about 300 billion watts) impinges on Earth's polar caps, generating spectacular auroral displays. It has been only withm the last 5 years that we have been able to obtain a view of the aurora on a global scale using an unager on the Dynamics Explorer satellite. Despite many efforts, very little Is known yet about how the energy enters the magnetosphere and circulates through the system, or where it is stored. Also little- understood are the physical processes that heat the solar wind plasma to several times its original temperature, and the eventual delivery of this energy into the high-latitude atmosphere. We are just entering a period that wiD generate sufficient data to allow a detailed understanding of Earth's magnetosphere and its environment. THE UPPER ATMOSPHERE There is an outer gaseous envelope surrounding aD the plan- ets (except Mercury) add some planetary satellites in our solar system. Such atmospheres absorb and redistribute globally the variable components of the solar energy falling on them. Plan- ets with strong magnetic fields have dynamic neutral gas plasma interactions associated with auroral activity and the convection of plasma around the planet. These processes are particularly prominent on Earth and Jupiter and have a strong influence on the global structure of the upper atmospheres of these planets. On Earth, the upper atmosphere/ionosphere acts as the ~nter- mediary between the plasma-dominated magnetosphere and the bulk of the neutral atmosphere below. The region Is highly com- plex. Interacting dynamical, chemical, radiative, and electrical variations occur there that couple the magnetosphere and middle atmosphere. To understand how these coupled elements interact to produce the great variability characteristic of the system is one of the major problems in solar-planetary relations. For example, the three-dimensional circulation of the thermosphere changes during and following geomagnetic storms; yet the consequences of the change of circulation on the temperature, density, composition, and electric currents of the region are poorly understood. Ener- getic solar particles penetrate the middle atmosphere ~d produce chemical changes in radiatively important species such as ozone, but their global consequences are not fully appreciated. Deeper in the atmosphere, solar-induced variations in the flux of cosmic

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31 rays may produce variations in the electrical structure of the lower atmosphere, but the effects of these variations on Earth's global electric circuit are not understood. MAGNl0TOSPlIlDRES OF OTHER P[AN1:TS AND COMETS Several planets possess magnetospheres because they have planetary magnetic fields; other planets, such as Venus, interact with the solar wind and form a downstream cavity, prunarily through an interaction with the planet's ionized atmosphere. The largest known magnetosphere in the solar system is that of the planet Jupiter; its dunension on the sunward side exceeds 4 million km about 6 times the radius of the Sun. The magnetic tail of Jupiter extends to at least the orbit of Saturn, a distance of 700 million km. Jupiter's magnetosphere is unique in that its plasma consists principally of oxygen and sulfur ions that are the ionized effluents of volcanic eruptions on the satellite To. Comets also have rudimentary magnetospheres. International Cometary Explorer (ICE; formerly ISE~3) demonstrated this during its passage through the coma of the comet Giacobini-Z~nner at a distance of 7000 km on the ant~solar side of the comet's nu- cleus. The influence of the comet ~ the interplanetary medium was observed as far as a million kilometers away from the nucleus. The interaction of the coma of the comet with the solar wind appears to be yet another example of plasma processes that can occur when the hot solar wind impinges on the coo} atmosphere of a solar system object. The March 1986 encounters of the Soviet, European, and Japanese spacecraft with comet HaDey yielded fur- ther unportant advances in understanding cometary physics. CONNECTIONS OF SOLAR SYSTEM SPACE PHYSICS TO [ABORAIO1lY AND ASTROPHYSICAL PLASMAS Nearly 30 years of apace research have clearly shown that many of the physical processes observed in the Sun, in the solar WiDd, and at the Earth occur throughout the universe. Detailed analysis and understanding of these, made possible by clos~range and in situ observation, serve to shape our studies of more dis- tant astrophysical phenomena. The rudimentary magnetosphere of Mercury is smaller than that of Earth by a factor of 20, while

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32 the size of the magnetosphere of a pulsar is believed to be com- parable to the size of Earth's magnetosphere. By contrast, the magnetosphere of Jupiter is about 100 times larger than the mag- netosphere of Earth, with an angular diameter about that of the Moon, even though Jupiter is 2000 times as far away. Even the size of Jupiter's magnetosphere, however, pales In comparison to that thought to surround the radio galaxy NGC 126~approximately 100 billion km. Physical processes that take place in Earth's magnetosphere have also been observed ~ other plasma ~laboratories," ranging in size from Tokamal fusion devices (approx~rnately 5 m) to solar flare kernels (less than 1000 km), to entire galaxies. Since the 1950s, plasma physics has developed along two separate, yet in- tunately connected paths: space and astronomical studies, and thermonuclear fusion research in terrestrial laboratories. Both are necessary for a complete understanding of solar processes. NATURE OF THE FIELD A substantial advance In the field of solar and space physics will require a major effort in each of its subdiscipl~nes: solar and heliospheric physics, magnetospheric physics, cosmic-ray physics, and upper atmospheric and ionospheric physics. Passive observational techniques are crucial to this field. They utilize instruments transported into space in a variety of ways: on high-altitude balloons and rockets; on long-lived earth satellites; and on interplanetary and planetary spacecraft, including plan- etary orbiters and entry probes. Active plasma experiments in the ionosphere and near-E=th environment have also played a significant role. Such experiments utilize electron and ion guns (small particle accelerators) and the controlled release of bursts of gas. In addition, ground-based observations play a continuing and important role. Basic theoretical work and modeling calculations underlie the entire field, guiding observational work. A balanced program including all of these elements is necessary- for progress. CURRENT FLIGHT PROJECTS A first-order priority during the next several years is to main- tain the operation of several existing satellites and spacecraft.

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33 These include the Dynamics Explorer 1 (Dim), the Interna- tional Sun-Earth Explorers 1 and 2 (ISE~1, 2), the International Cometary Explorer (ICE), the Active Magnetospheric Particle Racer Experiment (AMPTE), the Solar Mesosphere Explorer (SME), the Pioneer Venus Orbiter (PVO), and the four outer- planet~heliosphere spacecraft Pioneer 10, Pioneer 11, Voyager I, and Voyager 2. All of these spacecraft are devoted wholly or par- tially to solar and space physics and are cont~nu~g to provide valuable data. They have special importance because of the lim- ited prospect for new missions during the next several years. The August 1989 encounter of Voyager 2 with Neptune is an event of particular interest. PROSPECTIVE P1~1995 MISSIONS The Galileo orbiter/probe to Jupiter, the solar polar orbiter (Ulysses), the Upper Atmosphere Research Satellite (UARS), and the International Solar Terrestrial Physics Program (ISTP) are the most unport ant new elements of the solar and space physics program from now until 1995. Tm addition, plasma and plasma wave instruments, and an electron gun on a Shuttle or free-fly~ng mission In Earth's ionosphere are planned for this period. Also, the Mars Observer (MO) spacecraft wiD carry a magnetometer for improved study of the magnetic field of Mars. Galileo and Ulysses are really for flight, but their launching dates have been tong postponed and are still uncertain. WARS, ISTP, ~d MO launches will probably tale place in the early 1990~. A substantial portion of the Gadileo instrumentation ~ in- tended for second-generation study of Jupiter's magnetosphere. The Ulysses mission represents a pioneering effort to observe the interplanetary medium, solar energetic particles, cosmic rays, and the Sun's atmosphere at high solar latitudesall classical phenom- ena In interplanetary and solar physics. The goal of the UARS program is to understand processes that control the structure of Earth's stratosphere and lower thermosphere. This includes the ozone layer and its response to nature and artificial perturbations. ISTP is being developed jointly by the United States, Japan, and the European Space Agency. The main objective ~ to develop a comprehensive global understanding of the generation and flow of energy from the Sun through the interplanetary medium and into Earth's space environment. ~ addition, ISTP seeks to define

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34 the relationships between the physical processes that link differ- ent regions of this dynamic system. Finally, the High Resolution Solar Observatory (HRSO), with a spatial resolution of about 70 km, will provide initial data on plasma-magnetic field interactions related to the solar dynamo and energy transport In the solar atmosphere. 1~:COMMENDED PROGRAM: POST-1995 The steering group has identified a number of forward-Iook~g programs that will address basic scientific problems in solar system space physics. Some of these have been the subjects of prior spacecraft and mission design studies. 1. Ultraviolet and X-Ray Telescopes of the Highest Practical Resolution. Such instruments are required to investigate smaD- scale (1 to 100 km) processes associated with transient and turbu- lent regions. The evolution and dynamics of such small structures are believed to be essential to the understanding of large-scale structures such as active coronal regions, flares, and the origin of the solar wind. 2. Solar Probe. This is an exploratory mission, contemplated for the m~-1990s, to investigate the nearby neighborhood (al- titude of approximately I.9 niillion km) of the Sun. The basic scientific goals are to explore the solar atmosphere, which is now known to us only through remote unages. It wall also investigate the sources of the solar wind. The great technological challenge here will rest with the development of a heat shield capable of withstanding the high temperatures encountered as the spacecraft approaches the Sun. 3. Other Advanced Spacecraft. Some of these are in the con- ceptual phase: viz. the Earth Observing System (EOS), the Solar Terrestrial Observatory (STO), and the Advanced Solar Observa- tory (ASO). 4. Advanced Minions to the Outer Planets. Projects in this category that have been the subject of specific workshops are: the Cassini Mission, involving a Saturn orbiter for detailed, synoptic studies of Saturn's magnetosphere and other planetary purposes, and a Jupiter Polar Orbiter (]PO) as a successor to Galileo. 5. Intersteliar Probe (IP}. Ib this project, planned for the turn of the century, a spacecraft would escape the solar system at great

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35 speed (about 80 km/s) and enter the local interstellar medium within 10 years. Such a spacecraft, if launched in the year 2000, would overtake the Pioneer and Voyager spacecraft (launched in 1972, 1973, and 1977) within 5 years and proceed ahead of them into the outer solar system. 6. Remote Sensing of Magnetospheric Plasma. This is idus- trated by the comprehensive imaging of auroras on a global scale by Dim. This technique, emphasizing selected energy bands in the ultraviolet and x-ray regions of the spectrum, has great future pm tential for remote sensing from locations at the Lagrangian points, on the Moon, and on high polar-orbiting satellites. An important new technique for remote sensing of magnetospheric processes by observing escaping neutral atoms has also been demonstrated re- cently and should be fully developed. Such investigations will advance our understanding of the entry and circulation of plasma within Earth's space environment something addressed prior to this tune principally by local measurements, often separated by long time intervals. In some sense, the magnetospheric problem Is the inverse of that of the Sun, for which we have global measure- ments but lack details about the local environment. 7. Active Experimentation. This is a valuable technique in the study of space plasmas. It involves the injection of gases, electromagnetic waves, or particle beams into the natural environ- ment and the observation of the resulting interactions. Some of the most significant results have come recently from the creation of artificial comets by the Active Magnetospheric Particle Pacer Explorer (AMPTE), a collaborative program between the United States, Germany, and Britain. Further experiments of this type, as wed ~ observation of the solar wind interaction with natural comets, are planned. Studies of dust-plasma interactions using active experiments may be valuable in helping to understand the formation of the solar system. In addition, it may be possible to perform basic plasma physics experiments in space, including stud- ies of plasma confinement in fusion systems, without the presence of walls. CONCLUSIONS The goals of solar system space physics address not only the basic physics of magnetized plasmas in the solar system, but also

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36 the complex energy transfer beginning at the Sun and propagat- ing through the interplanetary medium to the magnetospheres, ionospheres, and upper atmospheres of Earth and other planets. Because of its proximity, the Sun is the only star whose in- terior structure and atmosphere we can study at high resolution, thereby providing information about physical processes important to all stars. Magnetospheres, the magnetized plasma atmospheres of Earth and the planets, are now known to exist throughout the universe around pulsars, radio galaxies, and accreting stars. The study of plasma processes that regulate the structure and dynam- ics of planetary magnetospheres has contributed significantly to the development of basic plasma physics. Further, there are im- portant connections between solar and space physics and earth system studies. Variations in the solar output of radiation and charged particles have substantial effects on the magnetosphere, ionosphere, and upper atmosphere. Even small variations of the solar luminosity may affect Earth's weather and climate. Strato- spheric and mesospheric ozone responses to incoming charged par- ticles from solar flares are examples of such processes. Finally, the understanding of the space environment near Earth has direct practical aspects, aside from its research value. Space Is being used increasingly for scientific, commercial, and national security purposes. Space vehicles must function continu- ously in the near-Earth environment, subject to the influences of the Sun, the magnetosphere and upper atmosphere, and cosmic radiation. In addition, these elements of the space environment will become particularly important to humans should they at- tempt to spend long periods of tune in space. This is true not just of manned missions within the magnetosphere and the interplan- etary medium, but also on the Moon and possibly on Mars. An improved understanding of solar variability and the perils of solar flare radiation is mandatory. Substantial advances in our ability to operate space-based systems safely and reliably will result from the basic studies outlined in this chapter. (For a general statement On the presence of human activity in space, see Chapter 9.)