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Plasma Science: From Fundamental Research to Technological Applications 6 Space Plasmas INTRODUCTION Background Space plasma physics is the study of natural plasmas in the solar system and associated technological applications. It is concerned with the ionospheric and magnetospheric plasmas of Earth and the other planets; the physics of the solar plasma internal to the Sun, the solar corona, and the solar wind; the interaction of the solar wind with planets, asteroids, dust, comets, and eventually the interstellar medium; and technology applications ranging from electric propulsion to space "weather" predictions. This is a vast, multiscale, physical domain wherein there are large variations in plasma sources, average thermal energy, flow velocities, magnetic field strength, and other physical parameters. The results are a rich collection of plasma physical behaviors that provide important intellectual challenges to fully understand the underlying processes. It is important to consider the wide variations in physical conditions in terms of spatial dimensions. The largest is that of magnetohydrodynamic flow phenomena. These include the massive outflow of solar plasma as the solar wind, which, entwined with the solar magnetic field, sweeps outward past the planets to its mixing with the interstellar medium. On a smaller scale, within planetary atmospheres, more complicated plasma flows are driven by electromagnetic fields associated with the interaction of the solar wind with the planetary body and atmospheric heating caused by solar radiation. On an even smaller scale, microscopic physical processes are occurring within plasmas in space. For example, the selective accel-
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Plasma Science: From Fundamental Research to Technological Applications eration of electrons and ions in the high-latitude, high-altitude regions of Earth leads to the formation of highly structured streams of energetic charged particles that impact Earth's upper atmosphere, creating the aurora. Finally plasma technology devices such as electric propulsion and plasma contactors operate on a small size scale, establishing their own local boundary conditions and interacting with the nearby space plasma. Knowledge of the physical processes operative in all of these examples is an important goal of space plasma physics for several reasons. First, it provides us with an understanding in quantitative terms of the variety of interrelated complex processes acting to shape and influence our terrestrial environment. Second, parts of the space plasma environment may be prototypical of the astrophysical environment. Third, space phenomena lead to fundamental scientific questions relating to the behavior of plasmas under conditions that can be very different from those created and studied in terrestrial laboratories. Finally, knowledge of the science underscores the development of technological applications operating in or based on the space plasma environment. As a consequence, investigations of natural space plasma processes extend the frontiers of human knowledge, enabling broader physical understanding of plasmas within the context of their general behavior. Understanding Earth's plasma environment also has important practical consequences. Among these are an ability to model and predict ionospheric, magnetospheric, and interplanetary disturbances that could adversely affect ground-based communications, sensitive instrumentation in geosynchronous orbit, and the safety of astronauts participating in future interplanetary endeavors. Status The era of in situ exploration of space plasma physics began in 1946 with V-2 rocket "snapshots" of the terrestrial space environment and continues aggressively today. Measurement techniques include both direct sampling and space-based remote sensing. An excellent example of the latter is the global observation from space of aurora at UV and optical wavelengths, clearly delineating the dynamics of the auroral oval. The initial exploration of the terrestrial magnetosphere and ionosphere is now reasonably well complete, although there are still regions of the solar system that have not yet been explored at all (e.g., Pluto, the heliopause, the solar corona) and regions that have been seen only through brief flybys (e.g., Mercury, Uranus, Neptune). Emphasis now is shifting to the details of physical processes controlling these plasmas. The results of all modern theories and models have depended significantly on the progress of in situ observations. Ground-based remote sensing studies of space plasma physics have played an important role by providing long-term, localized observations and understanding. Incoherent and coherent radar observations of natural ionospheric
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Plasma Science: From Fundamental Research to Technological Applications phenomena provide information with good temporal and altitude resolution from a few physical locations, thus complementing spacecraft measurements that give good global coverage. Magnetic and optical observatories help in elucidating global current systems and local energy deposition rates. Ground-based measurements that "modify" the natural plasma in the ionosphere have provided information on the physics of a variety of plasma instabilities and related phenomena. Over the past two decades, our capability to numerically investigate the behavior of space plasmas has steadily improved. Models and simulations of the 1960s and 1970s evolved as a consequence of attempts to understand particular features of the solar-terrestrial environment (e.g., the composition and thermal structure of the atmosphere and ionosphere, the dynamics of interhemispheric plasma interchange, the coupled dynamics of energetic plasma in the magnetosphere and electric fields and currents in the magnetosphere and ionosphere, the interaction of the solar wind with the geomagnetic field, the formation of shocks in the solar wind, the propagation of solar and galactic cosmic rays in the solar wind, and the dynamics of magnetic reconnection). More recently, the ability to study plasmas on a microscopic scale has evolved through the use of various simulation techniques with supercomputers. These codes permit the investigation of various modes of plasma dynamics associated with internal energy and momentum transfer between the plasma constituents and plasma waves. Unfortunately, owing to limitations of computer resources, these studies are often limited in terms of their spatial and temporal resolutions. The last 35 years of satellite exploration and ground-based experiments, going hand in hand with theoretical modeling and simulation, have put us at a stage where the gross plasma morphology of the solar system is defined in an average sense. This large-scale picture is a synthesis of a relatively few observations that are localized and scattered in both space and time. The major task ahead in our studies of space plasma physics is to obtain the necessary information to be able to understand and elucidate the processes that control the behavior of these plasmas. This will require the use of sophisticated, multispacecraft missions, accomplishing direct and remote sensing observations as well as active perturbation experiments. Ground observations and experimentation will continue to provide important long term measurements. Both space-based and ground experimentation will have to be coordinated effectively. Advanced computational techniques will dramatically strengthen theoretical modeling and simulation.
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Plasma Science: From Fundamental Research to Technological Applications TOOLS FOR SPACE PLASMA PHYSICS Space-Based Techniques The mainstay of progress in space plasma physics has been in situ and remote sensing experiments from space. They provide the means for systematically monitoring large regions of space plasma. The inability to distinguish space from time changes in measurements from a single spacecraft underlies the future thrust toward flying constellations of identically instrumented and electronically coordinated satellites to study a given phenomenon or a limited region of space. The success of such efforts will depend strongly on the implementation of self-contained "smart" electronics to facilitate real-time complementarity and software techniques for onboard selection, digestion, and compaction of the plethora of data from multiple sources. Active experiment techniques are used to create a controlled disturbance and study its effect on the environment. (See Figure 6.1.) Active experiments have a broad range of objectives. These include (1) simulation of natural processes occurring in space plasmas, (2) measurement of physical properties such as reaction rates of atmospheric constituents and collisional cross sections, (3) use of space as a laboratory without walls to study fundamental plasma physics, (4) probing the natural environment as is done in experiments tracing magnetic field lines by electron beams, and (5) improving communication systems by studying the propagation of electromagnetic waves. For the study of space plasma processes the attraction of active techniques is twofold: there is no need to wait for a phenomenon to occur naturally, and the source characteristics are known and can be controlled. In this way, active experiments are similar to laboratory experiments except that the former have the advantage that the space plasma for most purposes can be considered boundless. The disadvantage is that it is difficult to obtain measurements with high spatial resolution. To remedy this problem, multiplatform experiments have become more common in recent years. Ground-Based Techniques Although the magnetosphere is an enormous region in space, we benefit largely from the dipolar origin of the field, which causes all the geomagnetic field lines of the magnetosphere to intersect the Earth, and most of them in the polar regions. This focusing of field lines provides a tremendous benefit observationally because arrays of ground-based instrumentation are relatively inexpensive to deploy and operate and they can provide important correlative data as well as a global context within which to interpret satellite data. Ground-based data also provide a long-term database that permits understanding of the secular variations and changes in the solar-terrestrial "climate."
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Plasma Science: From Fundamental Research to Technological Applications
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Plasma Science: From Fundamental Research to Technological Applications A variety of standard observatories provide continuous measurements of magnetic fields, ionospheric conditions, and solar activity. Also, a number of facilities operate only occasionally, such as the large incoherent-scatter radar facilities. Plasma Theory and Simulations Theory must develop a framework for interpreting observations of various physical systems. With this framework as a basis, quantitative analytic and numerical descriptions of model physical systems are constructed and refined through comparison with observations. Ultimately, the model physical systems must be sufficiently similar to the natural physical systems, and the numerical and/or analytic descriptions of these model systems must be sufficiently refined to provide a high level of predictability of the observed behavior of space plasmas. (See Plate 5.) An outstanding issue involving modeling and simulations is how to properly represent multiscale phenomena numerically. For example, how can one best incorporate anomalous transport processes occurring in boundary layers (such as shocks), which occur on both temporal and spatial scales that are microscopic, into meso- or macroscale numerical codes? The ultimate test of accomplishment in theoretical and simulation research must be the degree to which closure is achieved with ground and space experiments. Theoretical investigations may be conducted with the goal of explaining physical phenomena that have been observed and measured, and theoretical studies may be designed to lead to predictions testable in space plasma physics missions. FIGURE 6.1 (page 104, above): Example of an active space-plasma experiment designed to study the deposition of energetic electron fluxes in the atmosphere. This controlled experiment, which was flown on the Space Shuttle ATLAS-1 mission in 1992, used an electron beam and an optical imager to study the deposition of high-energy electrons into the polar atmosphere. Shown in the top panel is the artificial aurora generated by the beam (upper right in the figure) and a quiet auroral arc (left). The electron beam pulse was 1 s in duration. The camera viewed downward along the magnetic field direction, and the direction of motion of the Shuttle Orbiter was to the left. The width of the image at a height of 110 km is about 80 km. The bar is linear in optical intensity. The image in the bottom panel is the same as that in the top panel but taken at a later time. It shows the artificial aurora superimposed on a large auroral arc. (Courtesy of S. Mende, Lockheed Research.)
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Plasma Science: From Fundamental Research to Technological Applications Laboratory Techniques Historically, laboratory experiments related to space phenomena never have been supported significantly by funding agencies, the exception being specific technology efforts such as electric propulsion. One reason is that it is costly and difficult to build experiments wherein the magnitudes of critical parameters scale with their space counterparts, a necessary condition for the laboratory work to be relevant. However, as a result of advances in general technology (i.e., developments in computers, digitizers, and other hardware), as well as large strides in the design and improvement of plasma devices, it is now possible to perform experiments that were not possible 15 years ago. This increased sophistication allows the possibility of meaningful laboratory simulations. Laboratory experiments can probe a process with unprecedented detail and uncover effects that may not be easily detectable in space. (See Plate 6.) Laboratory experiments can address both local and global physics issues, the latter often determined by boundaries. In some cases, one can comprehensively analyze physical phenomena simultaneously from both global and local points of view. Furthermore, experimental devices may be rapidly configured to perform new experiments as ideas are developed. This can happen on the time scale of days or weeks. The hardware is reusable and flexible. Many different experiments can be performed on the same machine. In addition to processes directly related to space plasmas, laboratory experiments have provided important technological advances within the areas of spacecraft propulsion and spacecraft potential control. Propulsion devices such as ion thrusters and arcjets, and plasma bridges, such as the plasma contactor, are being tested and studied in space with diagnostics developed largely for ambient plasma observations. FUNDAMENTAL PROCESSES IN SPACE PLASMAS Summarized below are a few basic phenomena of wide significance. Wave-Particle Interactions The important role of plasma waves in the macroscopic transfer of energy and momentum in space plasmas has become clear as a result of complementary and strongly coupled experimental and theoretical investigations. Over the last few decades, spacecraft observations have provided valuable information regarding the plasma environments around the Earth, planets, Sun, and comets. For example, from a limited set of planetary wave observations, it has become clear that similar waves exist around all the magnetized planets. This would seem to set limits on the significance of anthropogenic effects in triggering natural waves in Earth's magnetosphere. By far the largest body of information on the role of waves has been accumulated in Earth's plasma environment. Space-
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Plasma Science: From Fundamental Research to Technological Applications craft observations have characterized Earth's magnetosphere as a collection of discrete regions with distinctive physical properties separated by well-defined boundaries. Strong plasma waves have been observed within the boundary regions, particularly the magnetopause and plasma sheet boundary layers. For the magnetopause boundary layer, this has important consequences for the entry of solar wind plasma into Earth's magnetosphere. Spacecraft have mapped out the locale and statistics of occurrence of most important classes of plasma waves in Earth's magnetosphere. Besides being important for boundary region dynamics, plasma waves provide the dominant loss mechanism for energetic electrons in the inner magnetosphere via pitch-angle diffusion and are drivers for the precipitation of ions and electrons into the lower atmospheric regions. Plasma waves are thought to play a principal role in heating ionospheric ions in the topside ionosphere. Upwelling of these heated ions along magnetic field lines and their subsequent trapping in the equatorial plane due to interactions with regions of plasma turbulence provide an important source of magnetospheric plasma. A wide variety of plasma waves participates in the complex energy transfer processes on auroral field lines. Plasma waves have been used as a diagnostic tool to obtain properties of the plasma from both ground-based and space-based systems. Plasma irregularities in the ionosphere occur with scale-size distributions covering tens of kilometers to fractions of a meter. These irregularities, which result from poorly understood instability mechanisms, are a major source for the disruption of high-frequency (HF) and extremely-high-frequency (EHF) communication systems. Charged-Particle and Plasma Energization Charged particles in plasma can be accelerated to high energies through a variety of mechanisms, some of which occur in nature or can be induced in suitably arranged space experiments. These include particle acceleration through resonance with quasi-monochromatic waves; stochastic acceleration resulting from resonance overlap due to large wave amplitudes or the presence of a finite spectrum of waves; acceleration by parametric processes, such as beat waves, Brillouin, and Raman scattering; and acceleration by electric fields that result from changes in macroscopic plasma morphology as encountered, for example, on auroral field lines. These phenomena are fundamentally nonlinear and extremely complicated, from both theoretical and observational points of view. Although measurements of electric and magnetic fields can be made with very high time (spectral) resolution, particle measurements are comparatively crude. For many purposes, the particle distribution function must be known to an accuracy that cannot be obtained with present-day technology. As an example, only two of the three velocity components of a distribution are generally known (perpendicular and parallel to Earth's magnetic field). Yet in many resonance inter-
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Plasma Science: From Fundamental Research to Technological Applications actions with waves, it is the unknown component (the phase coherence) that is the key to the interaction. Dust-Plasma Interactions Dusty plasmas are the most common type of plasmas in space. It is now believed that the long-term evolution of the dust and plasma environments is strongly coupled. The dust grains collect electrostatic charges from the plasma, and the evolution of their spatial distribution, size distribution, and lifetime can be determined by electrostatic forces and plasma drag. On the other hand, the dust can alter the plasma composition, density, momentum, and energy distribution, as well as the dispersion relations of the waves propagating in a dusty plasma medium. In the past decade, a growing effort (laboratory experiments and theory) has focused on problems related to dusty plasmas. We now understand the important processes that determine the charge of the dust grains and have learned the transport processes that shape the fine dust components in planetary rings embedded in magnetospheric plasmas. Magnetospheric perturbations were clearly shown to be responsible for the observed spatial distribution of small dust grains in the Jovian and Saturnian rings. Collective dusty plasma effects were suggested to explain the spokes (transient radial dust features on Saturn's main ring system) observed on Voyager images. The large scattering cross section of charged ice grains in noctilucent clouds is thought to be responsible for the observed anomalous radar echoes. The differential settling of bigger and smaller grains toward the midplane in the early solar system was suggested to cause spatial charge separation that might have resulted in large-scale electrostatic discharges. These lightning bolts could explain the existence of chondrules (small molten beads of rocks found in meteorites). The Critical Ionization Velocity Effect Investigations of the critical ionization velocity effect are an important part of space plasma science. The phenomenon involves the nonclassical ionization of energetic neutral atoms and molecules as they move through a background magnetized plasma. From laboratory studies and some space measurements, it is thought that when the center of mass energy of the neutrals rises above their ionization threshold, there is rapid ionization of the neutrals. This process apparently involves energization of the ambient electron gas by plasma waves associated initially with the transformation of a few energetic neutrals to ions. The newly born ions have considerable kinetic energy and heat the electrons through collective plasma processes. When sufficient neutrals are converted to ions, as might happen through charge exchange, for example, the energy density of the
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Plasma Science: From Fundamental Research to Technological Applications electron gas rises to the point where additional ionization of the neutrals ensues, and a flash ionization of most neutrals occurs. This phenomenon has great significance for models of young solar systems. To perform comprehensive measurements of the processes involved, it will be necessary to achieve the correct physical scale: the electron gas must be heated over a sufficiently large distance that its temperature can rise to the point where impact ionization of the neutrals becomes important to the overall system of interacting gases. Such experiments lie in the future and will require much more extensive supporting resources than have been possible with small free-flying satellites or rockets. Radiation Processes The topic of radiation processes is relatively new and involves detailed study of the production, transport, and absorption of microwave, infrared, and shorter-wavelength radiation in dense plasmas. However, its implication to the study of astrophysical systems is profound. The interaction of such radiation with matter involves individual molecules, atoms/ions, or electrons—not collective plasma processes. Clearly, radiation processes are of fundamental importance in transporting energy through portions of the Sun and of the Earth's atmosphere. In addition, radiation propagating freely from its source and from optically thick regions is the primary means by which remote sensing is accomplished. The opportunity to study fully coupled electromagnetic radiation with plasma dynamics in the space environment supplements the extensive work done in laboratory plasmas on similar problems. ACTIVE EXPERIMENTS Active experiments have a broad range of objectives. The techniques used in active experiments include four main categories: (1) injection of plasma and neutral vapor; (2) injection of energetic beams of neutral particles, ions, or electrons; (3) wave injection from ground based systems of acoustic waves and electromagnetic waves in the very-low-frequency (VLF) and HF bands, or injection from space vehicles of VLF, HF, and microwave radiation; and (4) use of the spacecraft as a disturbance to study spacecraft wake, vehicle charging, ram glow, or the electromagnetic effects of tethered systems. Plasma and Neutral Mass Injections The natural space environment can be modified by the introduction of foreign gases and plasmas to induce or enhance local processes. These include changes of the local ion composition, reduction of the local electron density, changes in the charge state of ions, changes in the average energy of the local
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Plasma Science: From Fundamental Research to Technological Applications plasma, and the plasma's chemical nature. This permits study of various processes occurring within ionospheric plasmas, including production, loss, and transport. It is also a way of creating unstable environments that evolve in interesting and new ways not normally found in the natural environment. The possibility of creating a large-scale ionic plasma (positive and negative ions dominating the overall composition) is both interesting and important in that it allows new processes to become dominant in plasma behavior. Such experiments are difficult, if not impossible, to perform in terrestrial laboratories. Pulsed plasma beam or contactor experiments, where a dense plasma cloud is released into the ambient medium, can give information on plasma transport. The plasma cloud expands and distorts in response to its internal diamagnetic structure as well as the external flow field. This expansion sheds light on the fundamental plasma physics of high-beta plasma clouds, such as occur in the magnetotail, as well as the nature of the transport process when the cloud is diluted. Such experiments are conceptually similar to those already under way in ground-based laboratories, with an important exception. By using pulses of sufficient density and duration, it is possible to create steady-state diamagnetic plasma regions near the source. Information about the various processes acting in such an unusual plasma configuration is an important step toward understanding a new regime of plasma physics. Particle Beam Experiments Particle-beam experiments have been conducted from many sounding rockets, satellites, and space shuttle missions. Objectives in these experiments have been (1) to map the geomagnetic field structure and parallel electric fields by observing echoes of beam electrons from the magnetic conjugate mirror point or from electrostatic structures along auroral field lines; (2) to study auroral processes, such as optical emissions and wave turbulence in auroral particle beams; (3) to stimulate electromagnetic and electrostatic wave excitation; (4) to create suprathermal electron tails; (5) to observe the interaction of the particle beam with the neutral gas in the vicinity of the source payload; and (6) to study spacecraft charging and neutralization. Wave Injection Experiments Space-based wave injection experiments make use of a number of different techniques to launch waves into the plasma. Topside sounders rely on the excitation of plasma resonances. Transmitters have been used on a number of sounding rockets. Finally, modulated electron beams have been used as ''virtual antennas" on STS-1 and the Spacelab-2 shuttle missions and on numerous sounding rocket experiments. Waves have been detected to a distance of a few kilometers. In these experiments, receivers have been located on the transmitter platform, on
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Plasma Science: From Fundamental Research to Technological Applications a subsatellite, or on the ground. Objectives include the study of antenna properties, near-zone electromagnetic field studies, wave propagation, and wave-particle interactions. Ground-based HF wave (megahertz-range) injections are launched from powerful radars into the ionosphere under quiet conditions, during magnetic storms, or in conjunction with chemical releases from active experiments. A large number of plasma processes can be studied in this way: focusing or defocusing of the beam, interference with communications, heating of the plasma, generation of suprathermal electron fluxes and airglow, instabilities including self-focusing, parametric interactions and strong Langmuir turbulence, generation of plasma irregularities, focusing by chemical releases, and effects of a hierarchy of heater thresholds. In addition, pulsed HF heating is used to modulate the auroral electrojet current for extremely low frequency (ELF) or VLF wave generation. VLF wave experiments study wave-induced particle precipitation, earth-ionosphere wave guide modification, stimulation of VLF emissions, direct D-region heating, and ELF-modulated VLF to produce a polar electrojet antenna. Strong acoustic waves from explosions generate gravity waves and acoustic shock waves that couple into the plasma through collisional interaction. Vehicle-Environment Interactions A space vehicle perturbs the environment in a number of ways. Out-gas clouds, fluid dumps, and thruster firings interact with the ambient plasma much as the neutral gas injections described above. In addition, the structure itself creates a wake in the plasma, in particular for orbiting platforms, for which the spacecraft velocity generally is larger than the ion thermal velocity. Surface glow induced by neutral atmosphere interactions with the spacecraft surface has been studied in the case of the space shuttle. High-voltage power systems and their interaction with the ionosphere were studied in sounding rocket experiments. Objectives were to study the plasma sheath, the charging levels, and the steady-state currents in the ambient plasma. Processes associated with the physical contact between plasmas and exposed surfaces in space are an important practical aspect of many advanced scientific and technological space systems. For example, the ability to draw electron current from magnetized space plasmas is an essential feature of plans for power-producing electrodynamic tether systems. Charging of dielectrics in the vicinity of high-current beam experiments is similarly an important concern. It is striking to realize that while basic issues of plasma sheaths and current extraction have been known for more than 50 years, we still lack fundamental knowledge of the processes involved, especially at high voltages and currents. Relatively simple experiments, such as measuring the voltage-current collection curves for magnetized plasma, have yet to be done for ranges of parameters in which large-amplitude plasma waves play an important role.
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Plasma Science: From Fundamental Research to Technological Applications 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.
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Plasma Science: From Fundamental Research to Technological Applications 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 satellites—Geotail, 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 perspective—but not the proximity—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
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Plasma Science: From Fundamental Research to Technological Applications 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 structures—loops, prominences, holes, flares, mass ejections, and so on—in 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,
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Plasma Science: From Fundamental Research to Technological Applications 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 Space Physics, 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.
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Plasma Science: From Fundamental Research to Technological Applications In Situ Experiments The previous section concentrated on passive observations of natural processes acting in space. It is also possible to conduct in situ, or active, experiments whereby artificial injections of charged particles, neutral gases, or electro-magnetic waves are used to alter natural processes or to stimulate new processes in the ambient plasma medium. It is possible to contemplate a rich selection of potential in situ experiments capable of exploring new areas of plasmas in space. NASA is the principal sponsor of such work, but for the past four years, as a matter of policy, NASA has restricted its funding to those projects that explore natural processes, rather than artificially induced behaviors. However, since many of the results of the latter types of experiments have important implications for plasmas in different space environments, it is hoped that this policy will be reviewed. Here we give brief outlines of some possible in situ experiments that have special merit. Space vehicles offer the promise of performing three-dimensional experiments in unbounded plasmas with varying mixtures of neutral gas. These can be done on a scale size that should make the instrumentation easy to build. In addition, the relevant time scales are microseconds or longer, which are easily measured and recorded. In spite of these advantages, plasma experiments in space have not been easy to perform. The principal reasons are that diagnostic instruments are difficult to place accurately and the space platforms that carry them may be big enough to interfere with the experiment. By using space platforms with suitable resources, it should be possible to investigate steady-state diamagnetic cavities in space plasmas. In this situation, the plasma effusion speed from its source can be made larger than the diffusion speed of the magnetic field. A complex region of low magnetic field is maintained by plasma pressure against the flowing ambient plasma and ambient magnetic field. This is an unstable situation, which opens the way to investigation of various types of instabilities. It is likely that these will reveal the presence of many new high-beta plasma-magnetic field interactions that depend on various plasma and magnetic field parameters. Magnetic field interactions, analogous to the solar wind-geomagnetic field coupling, can also be anticipated as the capability to construct and operate large magnets in space evolves. These experiments, involving a variety of plasmas and magnetic field configurations, will have relevance to a wide range of astrophysical situations. Terrestrial Observation Networks Support for the existing standard observatories, which provide the long-term monitoring of fundamental parameters of the upper atmosphere, ionosphere, and magnetosphere, is a key part of a scientific strategy that recognizes the importance of time series data relating to the geophysical environment. Optical, radar,
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Plasma Science: From Fundamental Research to Technological Applications geomagnetic, and other instruments provide an important view of external plasma processes, and data acquired simultaneously from many sites provide a basis for understanding many different manifestations of magnetospheric and ionospheric dynamics that are closely linked to the solar wind. Recommendations from the scientific community include increasing the number of operating stations, as well as modernizing them, to enable development of precise and high-quality databases. Lack of observations from the Southern Hemisphere, particularly digital data, is a serious problem. These data are necessary to understand the asymmetries that arise from summer-winter differences in the polar ionospheres and from asymmetries in the geomagnetic field itself. Digital data acquisition in the Antarctic is particularly important, and the Antarctic is the only region where stable instrument platforms can be easily placed at very high polar latitudes (in the polar cap). Equal emphasis must be given to global arrays and to dense regional arrays of instruments. Concern should be paid to including complementary instruments within arrays in order to achieve a rich source of fundamental parameters. Arrays must also be utilized to deconvolve the spatial and temporal aliasing of the data. This is a particular problem with the interpretation of data from a single station or spacecraft. Thus, the coordinated use of multiple stations and multiple instruments should become increasingly the norm in data analysis. Laboratory Experiments When a phenomenon has been identified by a spacecraft and the basic physics of it is not well understood, the laboratory is the ideal place to study it. The problems encountered in space observations of single-point measurements and nonrepeatability are overcome in the lab. A well-planned experiment can be carefully tailored so that it is repetitive in space and time. Plasma laboratory technology has advanced to the point that many experiments pertinent to space plasma phenomena can be performed. For example, in wave studies, waves can be made linear or nonlinear by the turn of an amplifier knob. Furthermore, these waves can be launched from one or more antennas and their fields mapped in the near and far zone. Beams can be introduced from localized sources, density nonuniformities can be repeatably produced, impurities can be added in known amounts at a given location, and plasma drifts can be created. Furthermore, measurements can be acquired at thousands of three-dimensional spatial positions and thousands of time steps during the interaction. This is impossible in space. Laboratory experiments can address both local and global physics issues (the latter are often determined by boundaries). In some cases, one can comprehensively analyze physical phenomena simultaneously from both global and local points of view. Furthermore, experimental devices may be rapidly configured to perform new experiments as ideas are developed. This can happen on
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Plasma Science: From Fundamental Research to Technological Applications the time scale of days or weeks, as contrasted with many years for satellites and several years for rockets. The hardware is reusable and flexible. Many different experiments can be performed on the same machine. The challenge to laboratory plasma science is to continue to develop technology in order to extend the range of physical phenomena that can be studied. It is now possible to fabricate microscopic detectors and antennae that are capable of making spatially resolved, in situ measurements of the electric and magnetic fields in the plasma, the electron and ion temperatures, the plasma potential, and the velocity distribution functions. Nonperturbing optical techniques, such as laser-induced fluorescence and optical tagging, are now well established. Other new techniques are time-resolved tomography, electron cyclotron emission spectroscopy, and the use of the motional Stark effect. Three-dimensional probe systems can move detectors (optical or electronic) anywhere within large devices, so that full space-time data sets can be acquired. Visualization software and three-dimensional-graphics computers make analyzing these data possible. Scientific areas in which laboratory simulation experiments can be carried out with current technology include properties of Alfvén waves, magnetic field line reconnection, wave-particle interactions leading to chaos, and current modulation of plasma conductivity. CONCLUSIONS AND RECOMMENDATIONS Space plasma physics, as the study of natural plasmas and associated technological applications, represents a vast multiscale physical domain with large variations in plasma sources, average thermal energy, flow velocities, magnetic field strength, and other underlying physical processes. As such, it represents an important regime for plasma science and technology and for our civilization. First, it provides us with an understanding in quantitative terms of the variety of interrelated complex processes acting to shape and influence our own terrestrial environment. Second, it affords the opportunity to observe at closer hand phenomena that may be operative in astrophysical situations. Third, space phenomena stimulate fundamental scientific questions relating to the behavior of plasmas under conditions that can be very different from those created and studied in terrestrial laboratories. And finally, space plasma science underlies the development of technological applications operating in or based on the space plasma environment. As a consequence, investigations of natural space plasma processes extend the frontiers of human knowledge, enabling broader physical understanding of plasmas within the context of their general behavior. Progress to date in understanding the space plasma environment has provided us with a broad picture along with some detail. However, many important details of physical mechanisms remain unanswered, including interdependencies between sources and physical responses. Successful investigation of this envi-
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Plasma Science: From Fundamental Research to Technological Applications ronment requires a coordinated and balanced approach utilizing in situ observations, active experimentation, theoretical modeling, ground observations, and laboratory simulations. The requirement for a high degree of synergism is an inescapable conclusion. The cornerstones of space plasma physics are observations carried out, analyzed, and interpreted in conjunction with complementary theory and modeling. Space plasma physics has historically developed in this mode. With the advent of new technologies, opportunities for further scientific understandings are nearly limitless. In studies similar to this one, the space community is currently examining future directions and independently identifying possibilities. The panel supports such efforts. The use of space as a medium for active experimentation has declined to the point of near extinction. This is unfortunate, since active experiments may elucidate natural processes and expand in a unique way our basic understanding of the plasma state. The panel recommends a reinvigoration of the active experimentation area. Meaningful laboratory experiments simulating space phenomena can now be performed in a number of different problem areas. Such experiments provide the opportunity to examine the relevant science in a controllable and reproducible manner; they are thus an important adjunct to highly transitory space observations and can hence serve as a vehicle for interpreting, substantiating, and/or planning the latter. Such laboratory experiments have been largely discredited in the past because they did not scale properly to space conditions, but that shortcoming has been circumvented by developments in technology. The panel recommends an initiative in laboratory experiments of sufficient magnitude to establish a small interactive community.
Representative terms from entire chapter: