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SPACE PLASMAS 112 Much of our present knowledge of plasma sheaths comes from laboratory measurements. In the case of electron current collection, this has imposed severe limitation on the scale of phenomena that can be studied. Because the total number of electrons in a given plasma chamber is limited, laboratory measurements of electron current are limited in time and current density to very small values. Measurements in space offer a far better situation since in space one can place the collecting anode in an essentially unbounded medium. FUTURE PLANS AND OPPORTUNITIES In Situ Observations The major task ahead in our studies of naturally occurring space plasmas is to obtain the information necessary to understand and elucidate processes that control their physical behavior. This will require the use of sophisticated, multispacecraft missions containing the latest technology in direct and remote sensing instrumentation. The technology to carry out these studies exists today. We now know how to construct rugged, reliable, and lightweight instruments capable of making three-dimensional, high spatial and temporal measurements of particle fluxes. We have demonstrated that we can make detailed measurements of electric and magnetic wave phenomena and have had great success in making remote optical measurements from UV to microwave frequencies. There are plans to make some of these important measurements in the decade ahead, using relatively small, as well as larger, spacecraft programs. Some focused studies can be carried out with a single low-cost spacecraft. An example of such a mission is the Fast Auroral Snapshot (FAST) Explorer. It is a relatively low-cost mission planned to be launched in 1995. Its aim is to study the plasma microphysics of the terrestrial auroral zone, and it will make very- high-resolution measurements triggered by certain preprogrammed signatures. The possibility of even cheaper, but still highly sophisticated missions, using leading-edge civilian and military dual-use technology, is currently under study. The microelectronics revolution has enabled the design of small, fast, smart, less expensive instruments, compared with the standards of a decade ago. Microprocessors, use of higher-order software languages such as C++, and specialized semiconductor chips, which enable digital signal processing, analog- to-digital signal conversion, and other operations, can be built into instruments, providing wide flexibility and speeding up changes in operating modes and other functions. Specialized analog systems under digital control permit rapid and accurate changes in voltages, currents, and other important aspects of instrument operation. As a consequence, aperture sizes have shrunk toward theoretical limits, detector systems have become miniaturized, detector efficiencies have become high, power consumption has become very low, and data rates are fast enough to challenge every satellite or suborbital rocket system designer.
SPACE PLASMAS 113 Furthermore, the development of microprocessor systems capable of controlling all aspects of remote experiments has opened the way to new concepts of experiments, enabled by high data bandwidths and precise timing of the process or events under consideration. The combination of rapid switching of instrument modes, linked to high bandwidth data acquisition, and the ability to analyze data onboard the instrument platform and alter the course of the data taking, is a feature that is not fully in place but that opens the way to much more precise observations of plasma phenomena in space. However, many, if not most, of the major outstanding questions in space plasma physics require sophisticated and coordinated multiple satellite missions to provide the much needed ability to distinguish between spatial and temporal changes. This is not a new concept. The Global Geospace Science (GGS) Program is a part of the International Solar-Terrestrial Physics Program (ISTP) and consists of three satellitesâGeotail, launched in 1992, Wind, launched in 1994, and Polar, to be launched in 1995. The planned separation distance of these satellites is very large; thus, their mission is to study long-range correlations. Cluster, a European Space Agency (ESA) program, planned to be launched in 1995, is the first constellation mission. Four essentially identically instrumented satellites are planned to fly in a tetrahedral formation, with variable separation, which at times will be as small as a few ion Larmor radii. Another mission in the study phase, the Grand Tour Cluster (GTC), is aimed at studying low-latitude magnetospheric structures even smaller than an ion Larmor radius. All recent large-scale programs, such as GGS and Cluster, have involved international cooperation. Such joint endeavors are both necessary and desirable. Involvement of international partners not only decreases the cost involved for the participating nations, but also ensures that the best available technology is used and the best scientists are participating, thus enhancing the scientific return. Careful coordination of ground-based observations with satellite-based measurements also will lead to significant increases in the scientific return from such programs. Solar physics in general, and solar plasma physics in particular, are different in the sense that our knowledge to the present has been obtained largely from Earth or Earth orbit. The perspectiveâbut not the proximityâ changed in the fall of 1994 with the passage of the Ulysses spacecraft high above the Sun's south pole at a radial distance of about 2 au (where 1 au is the Sun-Earth distance). Despite the resultant limitations of poor spatial resolution, we infer strongly that a rich variety of plasma phenomena are occurring and that plasma physics is complementary to nuclear physics in determining solar structure and behavior. The Sun is a source of magnetic field, which probably is generated by the interaction between its differential rotation and MHD convection in its interior. The body of the Sun supports a plethora of waves, which manifest themselves through surface oscillations, whose study has given rise to the field of helioseismology. The magnetic field reaching the surface, rather than being
SPACE PLASMAS 114 uniformly distributed, is concentrated in small flux tubes, many having loop topology, so that the solar corona, as viewed in x-rays (see Plate 7), is a veritable archive of plasma structures: sunspots, fibrils, prominences, spicules, holes, bright spots, and so on. The corona is a dynamic region, the source of both a continuous solar wind and, from time to time, blobs of localized, energetic plasma flow that drive shock waves ahead of them as they propagate outward toward the planets. Such coronal mass ejections (CMEs) usually cause ground-based electromagnetic disturbances when they hit the terrestrial magnetosphere. Solar flares are associated with many CMEs and are one of nature's most observable examples of particle acceleration. Radiation spanning the electromagnetic spectrum is generated either directly by wave-particle processes or secondarily by energetic particles interacting with chromospheric material. In addition, flare-related relativistic electron beams propagating into the solar wind produce there characteristic (Type III) radio waves by processes that are generally thought to be highly nonlinear in nature. Understanding of plasma activity on the Sun would undoubtedly prosper from a high-resolution, FAST-type mission. That is impossible because of the distant and more hostile environment. The closest approximation is the Solar Probe spacecraft, currently under study, which would make a one-time pass within three to four radii of the nominal surface. Besides providing scientific insight through in situ observations, Solar Probe presents obvious technical challenges in the area of thermal engineering. In the meantime, the ESA Solar Optical and Heliospheric Observatory (SOHO) is being prepared for a 1995 launch, with U.S. participation in the instrument complement. The scientific objectives of SOHO are to study localized plasma structuresâloops, prominences, holes, flares, mass ejections, and so onâin the solar chromosphere, transition region, and corona by spectroscopy and imagery of their electromagnetic emissions at UV and visible wavelengths and, at the same time, to monitor derivative solar wind effects via onboard particle measurements. Additionally, data from instruments that measure fluctuations in solar brightness and coherent, long-wavelength oscillations of the solar disk, so called helioseismology, may shed light on processes occurring in the solar interior. Cassini, another mission in an advanced state of development, is currently being developed as a mission to Saturn scheduled for launch in 1997. Using a new technique, one instrument will be able to form a two-dimensional image, providing the direction of arrival of energetic neutral atoms formed via charge exchange with energetic ions. The results will enable the measurement of the spatial extent and energy composition of the large plasma zones surrounding Saturn and its satellites. Much future work in planetary science will focus on waves and instabilities in naturally occurring dusty plasmas. The Ulysses, Galileo, and Cassini missions will fuel more interest in this field. Data from dust detectors, imaging,
SPACE PLASMAS 115 plasma and plasma wave experiments, magnetic field measurements, and so on will be used to understand dusty plasmas in planetary magnetospheres and in the interplanetary medium. An important new mission to study the ionized and neutral upper atmosphere of Earth is slated for a new start within the next year. The Thermosphere-Ionosphere-Mesosphere Dynamics (TIMED) mission will carry a variety of instruments designed to probe complex interactions affecting the behavior of Earth's atmospheric regions lying above the stratosphere. The upper atmosphere has a factor of 10 greater response to global warming than the lower atmosphere, and can serve as an indicator of subtle changes that may be either anthropogenic or externally driven. TIMED will study this altitude regime, which experiences coupling between neutral and plasma constituents, and where competition between solar irradiance variation and plasma processes such as joule heating is important. Plans for deployment of an Earth Observing System as part of NASA's Mission to Planet Earth should incorporate as a component of the program the study of plasma coupling to the neutral atmosphere. A major difficulty with space satellite constellation experiments is that differences in satellite altitudes lead to different orbital periods. Coordinated local observations thus become a matter of occasional opportunity, and a concentration of observations from different satellites at one time will rapidly decay to widely dispersed observations over times of a few minutes. NASA is developing a new way to obtain coordinated measurements over distances up to several hundred kilometers. This involves the use of long tethers connecting individual satellite platforms together. In a static configuration, the instrument string (looking much like a deep-sea string of acoustical sensors) is deployed along a vertical direction. The entire system moves with a constant, common angular velocity with respect to the Earth. This results in the possibility of obtaining simultaneous plasma and atmospheric data over a wide range of altitudes. Such a system could be used to observe the high-altitude acceleration zone for auroral electrons, the possible presence of horizontal plasma shear in large-scale plasma convection in the polar caps, or the behavior of aurora plasma in the regions of atmospheric excitation. A substantial number of other missions exploring the behavior of space plasmas are now being planned by the U.S. and international scientific communities, with the Solar-Terrestrial Energy Program (STEP) providing coordination of both ground- and space-based systems. As in the past, there is a strong sense of cooperation among the international participants. An extensive and specific evaluation of future space missions, to be entitled A Science Strategy for SpacePhysics, is currently in progress under the auspices of the NRC's Committee on Solar and Space Physics and Committee on Solar- Terrestrial Research. When completed, this study will be used by NASA in its planning of new missions over the coming decade.
