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5 Space and Astrophysical Plasmas INTRODuCTION Most of the observable universe is in a plasma state. Plasma regimes range from the dense cores of stars to the relativistic electron-positron plasmas around pulsars and include the vast, diffuse plasmas that fill the spaces between galaxies. Furthermore, many of the fundamental questions in space and astrophysics require plasma physics for their answers. These questions have been identified by NASA, NSF, and other organizations as among the central science questions of our time. How does the universe begin? (As a plasma, according to the popular big bang model that has been so successful for the past decade.) How are the planets such as Earth formed? (In disks of plasma around stars, according to one theory.) What is the nature of our own solar system and its planetary plasma environment? And what is the nature of the extreme plasma environment around black holes? In addition to serving a purely intellectual purpose—enabling an understand- ing of the universe in which we live—plasma physics has important practical im- plications for the interaction of satellites and humans with the space environment. Astronaut safety and spacecraft health issues require analysis of the plasma physics of radiation environments and payload charging. Accelerating the commercial use of space requires detailed knowledge of space weather. Although the science behind space weather has begun to move from the research community to the commercial and operations communities, it is still a formidable challenge to model the near- Earth space environment to the required level of quantitative prediction. The popular appeal of space and astrophysics—both the science itself and 12

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sPace astroPhysical Plasmas 1 and programs such as NASA’s Space Camp, the Hubble Space Telescope, and the Apollo program—helps maintain the national awareness of intellectual, scientific, and engineering endeavors. Space science and astrophysics thus play an important role in motivating future generations of scientists, engineers, and—specifically—plasma scientists. Our current understanding of many space and astrophysical observations is rooted in plasma physics, and continued progress requires a better understanding of fundamental plasma processes. Indeed, conceptual advances in all of the six key processes discussed in Chapter 1 are essential. But while plasma physics can be considered a tool for space physics and astrophysics, the relationship is increas- ingly a two-way street. Space and astrophysical observations uncover dramatic and exotic new plasma physics regimes for study and provide detailed data to illuminate fundamental plasma processes. The diversity of plasma regimes encourages broad- based data analysis and innovative theory. In many cases, it also enables a search for new basic plasma processes that can be extracted from the specific parameter regimes and explored on a more fundamental basis. The space plasma and astrophysical plasma physics communities are at a criti- cal juncture. Instead of leaning on the laboratory plasma experience for guidance, they are pioneers in investigating new plasma physics regimes. How can the best progress be made in this new environment? Clearly, the goals of space physics and astrophysics are to understand the universe broadly, not to understand the specific details of plasma science. It is not obvious, however, where to find the most ef- fective balance between fundamental plasma understanding on the one hand and the application of existing knowledge to particular objects on the other. Both are needed for progress on the broad goals of space physics and astrophysics. It is im- portant to recognize and exploit the intimate links between plasma science in the laboratory and plasma science in space and astrophysics. Such cross-fertilization requires close communication and coordination between communities to enhance the flow of information in all directions. The next section highlights progress and prospects for three research topics in space and astrophysical plasma physics: the origin and evolution of structure in a magnetized plasma universe; particle acceleration throughout the universe; and the interaction of plasmas with nonplasmas. The chapter concludes with a sum- mary of challenges for the next decade and recommendations for meeting these challenges. RECENT PROgRESS AND FuTuRE OPPORTuNITIES Space and astrophysical plasma physics includes systems ranging from the Earth’s mesosphere through the solar wind and heliosphere (Figure 5.1) and out through plasmas on the scale of the universe as a whole. The physical conditions

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Plasma science 14 FIGURE 5.1 TRACE image of the solar corona, illustrating some of the science questions presented in this chapter: structured plasmas protruding from the surface of the Sun with particle acceleration dur- ing solar flares, and interaction between the collisionless solar corona and the collisional points near the photosphere of the Sun. Courtesy of Transition Region and Coronal Explorer (TRACE), a mission of the Stanford-Lockheed Institute for Space Research and part of the NASA Small Explorer program. in space and astrophysical plasmas vary enormously, both in terms of the absolute densities and temperatures and in terms of the dynamical importance of processes such as collisions between particles. Consider a few examples of the relevant physical conditions. In Earth’s atmosphere, temperature and gas density range from approximately 270 K and 3 × 1019 particles per cubic centimeter at the surface of Earth (where matter is neutral and not a plasma) to 180 K and 1,000 electrons per cubic centimeter in the partially ionized collisional mesosphere. From the center of the Sun to the solar wind at Earth, the temperature decreases from ~5 million K to ~105 K and the density decreases from ~1026 particles per cubic centimeter to ~1 particle per cubic centimeter. On astrophysical scales, the range of physical conditions is even more extreme, with temperatures reaching ~109 K around some black holes and neutron stars and gas densities as small as ~10–4 particles per cubic centimeter in the space between

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sPace astroPhysical Plasmas 1 and galaxies (roughly a billion times more rarified than the best vacuums created in laboratories on Earth). Magnetic field strengths range from ~1015 G for the most strongly magnetized neutron stars (a billion times greater than the largest sustained laboratory magnetic fields), to ~1 G for Earth, to ~10–6 G in galaxies like our own. Although the latter magnetic field strength may sound weak in absolute terms, the magnetic force on the gas in galaxies is as important as the vertical gravity in the disc of a galaxy. Studying plasmas over such a range of parameter space is greatly helped by the fact that the underlying plasma physics is often indifferent to the absolute tem- perature and density of the plasma but instead depends only on key dimensionless ratios. For example, the dynamics of a plasma is typically much more sensitive to the ratio of the magnetic energy density to the thermal energy density of the plasma than it is to the absolute value of either energy density alone. Thus the strongly magnetized coronas of a star and a galaxy have much in common even though the galaxy’s magnetic field is at least six orders of magnitude smaller. In addition to the similarities in key dimensionless ratios, similar physical processes are also at work in a wide range of space and astrophysical environments. One of the goals of this chapter is to highlight three technical questions that cut across a wide range of problems: (1) To what extent is plasma science independent of the regime? (2) When is the coupling between small and large scales important? (3) How does nonideal plasma behavior influence dynamics? With such a diverse range of plasma regimes to study, an exhaustive account of the progress and challenges in space physics and astrophysics is impossible. Instead, examples of efforts at the intersec- tion of space and astrophysics with plasma physics are presented. In developing its analysis, the committee relied heavily on the excellent work of two previous NRC committees. In addition to hearing testimony from partici- pants in those studies, this committee paid close attention to three earlier reports, Plasma Physics of the Local Cosmos (2004), The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics (2003), and Astronomy and Astrophysics in the New Millennium (2001). Readers interested in a more compre- hensive discussion are strongly encouraged to consult these references. Finally, this report highlights only the most compelling research themes; discussion of specific opportunities in HED astrophysical science can be found in NRC report Frontiers of High Energy Density Physics: The X-Games of Contemporary Science (2003) and Frontiers for Discovery in High Energy Density Physics (2004), a report from the White House’s Office of Science and Technology Policy. What Are the Origins and the Evolution of Plasma Structure Throughout the Magnetized universe? The observable matter in the universe is predominantly in the form of magne- tized plasma. The largest volumes of such plasmas are in the intergalactic medium

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Plasma science 1 (e.g., galaxy clusters) and the smallest such surround planetary moons. The origin of magnetic fields in such objects (galaxies and moons) is one of the central puzzles in plasma astrophysics and space physics. Equally important is the question of how the magnetized plasma influences the structure, both spatial and temporal, and evolution of the object under consideration. Clearly these questions are ultimately related to the process of magnetic self organization, one of the six key plasma pro- cesses highlighted in Chapter 1. Here the current understanding of magnetic field generation and its impact on the evolution of structure in the universe is reviewed, highlighting recent progress and suggesting directions for future research. The discussion starts with the largest scales of the universe as a whole and proceeds to smaller and smaller scale objects such as galaxies, stars, accretion disks, and the planets in our solar system. Plasmas and Magnetic Fields on Cosmological Scales It is not known when and how the universe first became magnetized. Although there are various theoretical arguments that small fields could have been generated primordially in the early universe (while it was entirely a plasma), there are cur- rently very few observational constraints on these processes. Aside from primordial theories, the leading idea for the origin of magnetic fields is that they are amplified and shaped from weak seed fields by the turbulent motions involved in structure formation. Weak seed fields can be produced by many mechanisms, including thermoelectric-driven currents. This mechanism is called dynamo action. Because the electrical conductivity of astrophysical plasmas is so large, the field remains nearly frozen in the plasma; the field lines move like threads stuck into the plasma, as they would in a superconductor. The field is thus stretched and amplified by the turbulent motions of the plasma. Although it is generally believed that dynamo action is responsible for the origin of magnetic fields in smaller gravitationally bound objects (e.g., stars, gal- axies, planets), its application to the largest structures in the universe is less clear. Smaller objects have the significant advantage that they amplify fields much more rapidly since they have shorter dynamical times and rotate faster. Fields amplified in energetic small objects such as accretion disks around black holes can be subse- quently ejected via outflows into the surrounding space. For example, observations of clusters of galaxies directly show magnetized outflows (“jets”) from the central black hole extending out into the intergalactic medium (see Figure 5.2, showing an x ray and radio image of Abell 400). However, such fields weaken when they are ejected into a larger volume, and it is not yet clear whether they can magnetize the vast volumes of intergalactic space. To understand the formation of large-scale structure, astrophysicists have em- ployed large-scale numerical simulations to model the collapse and clumping of

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sPace astroPhysical Plasmas 1 and FIGURE 5.2 This composite x-ray (blue) and radio (pink) image of the galaxy cluster Abell 400 shows two radio jets immersed in a vast cloud of multimillion-degree x-ray-emitting gas that pervades the cluster. The jets emanate from the vicinity of two supermassive black holes (bright spots in the image). The image is approximately 1 million light-years on a side. Courtesy of NASA/CXC/AIfA/D. Hudson and T. Reiprich and NRAO/VLA/NRL; based on data in D.S. Hudson, T.H. Reiprich, T.E. Clarke, and C.L. Sarazin, Astronomy and Astrophysics 453: 433-446 (2006). dark matter and gas. Given the complexities of this problem, most research to date has ignored the magnetic field and the fact that most of the matter in the universe is an ionized plasma. If, however, the field was formed early in the evolution of the universe (either primordially or by the first generation of stars and black holes), the magnetic forces may play a significant role in the subsequent evolution of structure

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Plasma science 1 in the universe. In addition, much of the plasma in the universe is relatively low density and hot (between 1 and 10 keV). The mean free paths of electrons and ions are thus quite large, and the transport of heat and momentum by the low colli- sionality plasma can have a significant influence on the behavior of plasma during structure formation. It is therefore expected that the plasma physics of structure formation will be a significant topic for research in the coming decade. Plasmas and Magnetic Fields on Galactic Scales As the universe expands and cools, galaxies form as plasma flows in toward the center of gravitational-potential wells established by dark matter. Magnetic fields in intergalactic space will be dragged in with the plasma, providing the ini- tial seed field for the magnetized plasma now observed to fill the space between stars in galaxies—the interstellar medium (ISM). The initial seed magnetic field is subsequently amplified and shaped by the complex physical processes occurring in galaxies. Outflows from stars (like the solar wind) and explosions of stars (superno- vas) can churn up the plasma in galaxies and also twist and amplify the magnetic field; similarly, the rotation of gas in a galaxy amplifies the galactic magnetic field. Through these dynamo processes, magnetic fields in galaxies are believed to ac- quire both a large-scale coherence such as that seen in Figure 5.3 and small-scale turbulent structure. Plasma and magnetic fields can also be ejected from the galaxy to form a galactic corona, analogous to the solar corona. Dense magnetized clouds of weakly ionized plasma in the ISM are often the sites of intense star formation, as clumps of gas collapse under their own gravita- tional pull. Understanding the physics of the ISM in detail is thus a key to under- standing how stars like the Sun form. Observations reveal that the ISM in galaxies is highly turbulent, with the random velocities often greatly exceeding the speed of sound. The energy source that maintains these motions is poorly understood and is one of the central problems to be addressed in the coming decade as numerical simulations improve and can be quantitatively compared to observations. Because of its enormous size, the gas (plasma) in galaxies is a useful environ- ment for studying some aspects of basic plasma physics. A particularly important example of this is that the spectrum of density fluctuations in the ISM of our galaxy is a k–5/3 power law over nine orders of magnitude in length scale. This is identical to the power law predicted and observed for unmagnetized (Kolmogorov) turbulence and yet the ISM is strongly magnetized. Recent attempts to understand this puzzle have led to significant advances in the understanding of the nature of plasma tur- bulence (a key process highlighted in Chapter 1). The resulting Goldreich-Sridhar theory, which has been confirmed in some respects by simulation, is an important breakthrough in the understanding of plasma turbulence and has a wide variety of applications to space and astrophysical plasmas.

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sPace astroPhysical Plasmas 1 and FIGURE 5.3 Galactic magnetism. Radio image of nearby galaxy M51, the “whirlpool galaxy.” Colors show the intensity of plasma emission and black lines show the direction of the mag- netic field inferred from the polarization of the emission (the length of the black lines is pro- portional to the degree of polarization). Cour- tesy of NRAO/AUI/NSF. Plasmas and Magnetic Fields in Accretion Disks The inflow (accretion) of matter toward a central gravitating object is one of the most ubiquitous processes in astrophysics and is responsible for forming much of the structure in the universe. During the accretion process, the gravitational potential energy of the inflowing matter is released in the form of radiation and outflows. When the central object is a black hole or neutron star, this liberation of energy is one of the most efficient ways of converting matter into radiation known in the universe. It is up to 50 times more efficient than nuclear fusion in stars. An understanding of the plasma physics of accretion is essential for solving a wide variety of problems—from the formation of stars and planets to achieving the long-sought goal of using observations of black holes and neutron stars to test general relativity’s predictions for the structure of space-time in the most extreme environments. In the next decade, observational techniques will enable direct imag- ing of plasma in the vicinity of the event horizon of massive black holes in several nearby galaxies. There are exciting prospects for seeing general relativistic effects in such observations, provided that the dynamics of the plasma around the black hole is sufficiently well understood.

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Plasma science 10 In the past decade, understanding of the plasma physics of the accretion process has advanced enormously. It was shown that a differentially rotating plasma is un- stable to generating dynamically strong magnetic fields, which redistribute angular momentum and allow plasma to flow inward. Experiments are being developed to study this magnetorotational instability and its nonlinear evolution in liquid metal experiments; indeed, it may already have been detected in a recent experiment. Numerical simulations have begun to study the time-dependent dynamics of disks, significantly improving on previous steady-state theories. In the context of accretion onto black holes, simulations have been carried out in full general relativistic MHD (see Figure 5.4 for a snapshot of the flow structure from such a simulation). Rapid progress is likely to continue over the next decade as the simulations incorporate more realistic physics and can be compared more closely to observations. Under certain conditions, the plasma flowing onto a black hole or a neutron star can be so hot and tenuous that the collisional mean free path greatly exceeds the size of the system, much like the solar wind. Initial progress has been made on FIGURE 5.4 The inner regions of an accretion disk around a black hole, as calculated in a general relativistic MHD numerical simulation. The black hole is at coordinates (0,0) with an event horizon of radius unity. The accretion disk rotates around the vertical direction (the axis of the nearly empty funnel region). Its density distribution is shown in cross section, with red representing the highest density and dark blue the lowest. Above the disk is a tenuous hot magnetized corona, and between the corona and the funnel is a region where there is ejection of mildly relativistic plasma that may be related to the formation of the jets seen in the earlier figure. Image based on work appearing in deVilliers et al. (2003), © American Astronomical Society.

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sPace astroPhysical Plasmas 11 and understanding how such a magnetized collisionless plasma accretes, but more work is needed on the dynamics of such low-collisionality accretion flows. In addition to providing a key observational window into black holes and neutron stars, accretion disks are also the sites of star and planet formation, as discussed later in the section on nonideal (dusty) plasmas. Plasmas and Magnetic Fields in Stars Most stars are sufficiently hot and ionized to behave as plasmas throughout most of their volume. Surrounding the star is a magnetized plasma environment— for example, the Sun has a hot plasma corona and farther out the solar wind. Loops of magnetic field emerge from the Sun’s surface (Figure 5.1). Periodic flares and eruptions of plasma release significant amounts of magnetic field energy in the form of heat, radiation (largely x rays), and accelerated particles. It is thought that the release of magnetic energy is a result of magnetic reconnection and is the dominant source of energy for the solar corona. (Magnetic reconnection is discussed in more detail in the next subsection.) In addition to this flaring near the surface of the Sun there is also extended heating out to distances of a few solar radii along open magnetic field lines. This heating is believed to drive away some of the coronal plasma leading to the solar wind. In the past decade observations with the SOHO satellite have provided direct constraints on the physical origin of this heating, implicating heating by very high frequency plasma fluctuations (near the cyclotron frequency). However, a detailed understanding of the origin of these fluctuations remains elusive. The Sun’s magnetic field, which is responsible for much of the activity in the corona and solar wind, is believed to arise by means of a dynamo driven by solar convection and rotation. The rotation profile of the solar interior inferred from observations of sound waves on the surface of the Sun (helioseismology) has provided strong constraints on the dynamo process. Large-scale numerical simu- lations have made significant progress in understanding solar convection and its effect on the solar magnetic field, but many features of the solar dynamo and solar structure (e.g., the magnetic field reversals of the Sun and the rotation profile in the solar convection zone) remain to be understood as the computations become increasingly realistic. An extreme analogue of solar flares is observed from a class of astrophysical ob- jects that occasionally produce large flares of gamma-ray radiation. It has now been confirmed that these flares arise from “magnetars,” neutron stars with the strongest magnetic fields of any known stellar object (roughly 1014 to 1015 G, compared to about 1012 G for more typical neutron stars and about 1 G for the Sun). Theoreti- cal arguments suggest that such magnetic fields may arise in a dynamo during the first 30 sec in the life of a rapidly rotating neutron star after it is formed from the

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Plasma science 12 collapse (and explosion) of a massive star. Magnetars appear to make up about 10 percent of the neutron star population, suggesting that for a reasonable fraction of the time, the formation of compact objects involves dynamically important mag- netic fields. Another class of astrophysical gamma-ray transients—long-duration gamma-ray bursts—has also been definitively linked to the explosions of massive stars (supernovas). These observations strongly motivate studies of supernovas, including the effects of magnetic fields. Such studies have just begun in detail, and significant progress is likely in the coming decade. Plasmas and Magnetic Fields on Planetary Scales The planets in our solar system are buffeted by the solar wind plasma that streams out of the Sun past the planets. This solar wind plasma demarcates the heliosphere. The interaction of the solar wind with the atmospheres and magnetic fields of the planets creates magnetospheres, plasmas that are trapped on the mag- netic field lines emanating from the planets themselves. In the local cosmos, the structure and evolution of the heliosphere of our Sun and the magnetosphere of the Earth are controlled and ordered by magnetic fields. They are a primary parameter of space weather, which has important consequences for satellites and humans in space. Thus understanding how magnetic fields are generated, transported, and dissipated is a fundamental problem in basic plasma science and of great impor- tance for describing magnetospheres. Three questions dominate current research: magnetic reconnection at boundaries, Alfvénic coupling and transport across magnetospheric regions, and planetary dynamos. Magnetic Reconnection. The breaking and reconnection of magnetic field lines is an important part of magnetic self-organization, which has significance for labo- ratory, fusion, and space plasmas. The basic process and the outstanding issues are described in the first section of Chapter 1. The prevalence of this research topic is a symptom not of repetition or redundancy in plasma science but of the underlying unity of the intellectual endeavor. As a physical process, magnetic reconnection plays a role in magnetic fusion, space and astrophysical plasmas, and in laboratory experiments. That is, investigations in these different contexts have converged on this common scientific question. If this multipronged attack continues, progress in this area will have a dramatic and broad impact on plasma science. At planetary scales, reconnection shapes and organizes the magnetic field of the planet and the solar wind. Significant reconnection occurs between field lines in distinct regions of the solar wind; field lines in the solar wind and the magneto- sphere at the magnetopause (on the Sun side of the planet); and in the magnetotail (on the side of the planet away from the Sun). Reconnection in Earth’s magnetotail releases magnetic energy explosively and initiates substorms—the excitations of the

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(a) (b) FIGURE 5.10 Auroral kilometric radiation (AKR) maser instability. (a) Energetic electron distribution function contours perpendicular and parallel to the magnetic field from FAST. This distribution is unstable to relativistic electron-cyclotron waves that are observed as AKR. Arrows indicate energy flow in the instability. (b) The frequency spectrum of the emitted radiation, electron energy distribution, electron angular distribution, ion energy fig 5.10 A and B distribution, and ion angular distribution versus time as seen by FAST. Courtesy of R.E. Ergun, University of Colorado, Laboratory for Space and Atmospheric Physics. 1 Lasndscape viefw

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Plasma science 14 is the dominant mechanism for particle acceleration in relativistic astrophysical plasmas. The understanding of magnetic reconnection in a relativistic environment has just begun; the development of such understanding, through theory and kinetic simulation, as well as the incorporation of that understanding into macro- scopic models, is a crucial requirement for advancing the modeling of relativistic environments. Significant effort has gone into extending the diffusive shock acceleration mechanism to the relativistic environment. Calculations have shown that large amplitude magnetic turbulence is required to provide sufficient scattering in the vicinity of the shock. In the last decade, direct simulation techniques have been ap- plied to the relativistic shock problem, for shocks both with and without upstream magnetic fields (see, for example, Figure 5.11). To date, relativistic shock simula- tions have yet to show solid evidence for significant particle acceleration, includ- ing no evidence for the high turbulence levels required in the phenomenological models. Deeper resolution of these issues awaits the rapidly improving ability to do three-dimensional simulations. FIGURE 5.11 Magnetic energy density in a relativistic collisionless shock, viewed toward the upstream direction; the shock propagates toward the lower right corner. The filamentary structure is due to the instabilities that generate the shock. Courtesy of A. Spitkovsky, Princeton University.

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sPace astroPhysical Plasmas 1 and How Do Plasmas Interact with Nonplasmas? The interactions of plasmas with neutrals, particulates, and boundaries is a field of study well illustrated by space observations. Many of the scientific issues in this area have parallels in low-temperature laboratory plasma physics. For example, spacecraft charging in plasmas is a complex technological problem with roots in laboratory and theoretical studies of sheaths (Box 1.1). Interactions of plasmas with neutral gases are important both at atmospheric boundaries and in the far heliosphere. Dusty plasmas appear throughout this entire report, with connec- tions to fusion, low temperature, and basic plasma physics (see Chapter 6). Dusty plasmas in space are a significant part of this field of study. In the heliosphere, dust from meteors, comets, and planetary rings provides a rich natural basis for the study of dusty plasmas. On even larger scales, the small admixture of plasma and charged dust in galaxies like the Milky Way strongly influences how stars and planets form. Recent progress in the basic physics of dusty plasmas is addressed in the same section. There are many fundamental open questions about plasma–nonplasma in- teractions. Is the mesosphere an active or passive part of atmospheric and climate change? What are charging and accumulation processes for particulates (charged dust)? How does ionospheric plasma physics mesh with atmospheric chemistry? What are the physics of mass loaded plasmas, partially ionized plasmas, and neu- tral atom–plasma interactions? How does the plasma physics change if the plasma is just one of many species present and is weakly (or strongly) interacting with them? What is the plasma physics (probe physics) of sheaths around charged spacecraft? Questions like these provide the opportunity to study nature but also promise insight into technological problems in fusion, industrial plasmas, and probe physics. Astrophysical Examples of Plasma–Nonplasma Interactions In many astrophysical environments, the interaction between plasmas and nonplasmas plays a crucial dynamical role. This is particularly true of the dense, relatively cold gas out of which stars and planets form. The majority of this cold gas is neutral atomic or molecular material that only indirectly feels the effects of the ambient electric and magnetic fields, via collisions with the comparatively rare ionized matter. One specific context in which these plasma physics issues have been extensively studied is the accretion disks present in sites of star and planet forma- tion. The same general issues that arise in this context also arise throughout the ISM of galaxies more generally and in the dense nuclei of galaxies where massive black holes form and grow. Planets, including Earth, form as gas and rocks collect together in the disk of dust and gas surrounding a newly formed star. The past decade has seen a

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Plasma science 1 revolution in our understanding of planetary systems, with the discovery of over 200 extrasolar gas giant planets (like Jupiter). Many of these planets are on rather elongated (eccentric) orbits close to their parent stars, in contrast to the massive planets of our solar system, which reside at large distances from the Sun on nearly circular orbits. The most plausible explanation for this difference is that the planets were formed at large distances but some slowly moved inward through interactions with their host accretion disk. The accretion disks out of which planets form are believed to be only weakly ionized (Figure 5.12). The plasma physics issues for this problem thus naturally evoke two general questions: FIGURE 5.12 The Voyager 2 spacecraft discovered the spoke structure on Saturn’s rings. These may be charged dust elevated above the larger ring bodies. Courtesy of Calvin J. Hamilton.

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sPace astroPhysical Plasmas 1 and • What is the actual degree of ionization in disks around young stars, and how is the coupling between the gas and the magnetic field maintained (if indeed it is)? • How does the accretion process proceed under low-ionization conditions, and what are the implications of the low degree of ionization for the mecha- nisms of star and planet formation and planetary migration? Heliospheric Dust and Neutral Interactions with Plasmas Progress in clarifying dusty plasmas will have a big impact on heliospheric physics. Both the heliosphere and the ISM are full of dust of all relevant sizes. Inter- stellar dust grains are present at all ecliptic latitudes throughout the plasma-laden heliosphere and in adjacent interstellar space, where they form about 1 percent of the ISM. Grains with an interplanetary origin are found in the ecliptic plane and isolated cometary streams. In studying the interaction between charged interstel- lar dust grains and the heliosphere, the goal is to understand the time-dependent and size-dependent filtration of interstellar dust grains in different heliospheric regions. During the first Jupiter flyby that deflected the Ulysses satellite into a circum- polar orbit, onboard dust detectors separated out two dust populations—small particles of jovian origin and grains with retrograde orbits, as expected for interstel- lar dust grains coupled to the interstellar gas flowing at about 26 km/sec through the heliosphere. Subsequent observations by Ulysses, Galileo, and Cassini found interstellar dust at all ecliptic latitudes. The plasma wave detectors on board the Voyager 1 and 2 satellites have detected micron-sized grains out to 85 AU in the outer heliosphere. Grain fluxes in the outer heliosphere are an order of magnitude higher than in the inner heliosphere. Some of the unsolved problems regarding the interaction between interstellar dust and the heliosphere are the following: • Understand the charging, filtration, and deflection of small charged grains as the grains cross the bow shock in the outer heliosheath regions and enter the heliosphere. • Understand the effect of merged interaction regions (turbulent regions in the heliosphere) on small-grain dynamics in the outer heliosphere, includ- ing grain charging and deflection. • Model the diffusion or streaming of grains with an ecliptic (planetary) origin toward higher latitudes, for all radial distances in the heliosphere. • Understand the differences seen between interstellar dust fluxes at Voyager 1 in the outer heliosphere and those measured in the inner heliosphere and at high latitude by Ulysses and other spacecraft.

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Plasma science 1 Timely answers to these questions will help us to understand the size and mass distributions of small interstellar and interplanetary dust grains that have been returned to Earth by STARDUST, which brought dust samples from the comet Wild 2 back to Earth, as well as the expected grain fluxes from future dust observatories in space. Mesospheric Dust and Collisional Plasmas Earth’s mesosphere starts about 40 km above Earth’s surface, where the at- mosphere is neutral, and ends 80 km above the surface, where the gas is partially ionized (Figure 5.13). This region provides an excellent laboratory to study funda- mental low-temperature plasma physics issues. These issues are of great importance in understanding possible changes in our atmosphere. Indeed, predictive modeling of the mesosphere requires a better understanding of the plasma science. Here the focus is on two interrelated plasma issues that are being studied: FIGURE 5.13 Noctilucent clouds. These beautiful highflying clouds form at heights of 80 kilometers above sea level or more and are thought to be made of ice forming around mesospheric dust. Because these clouds reflect light very weakly, they are only visible just after nightfall as in this photo where the noctilucent clouds are easily spotted because they are the only clouds high enough to reflect light from the setting sun. Courtesy of Pekka Parviainen, © 2004.

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sPace astroPhysical Plasmas 1 and • The transition from a collisional to a collisionless plasma environment as a function of altitude and • The interaction of the mesospheric gas and plasmas with dust and aerosols. Mesospheric chemistry is highly dependent on the plasma/gas conditions; however, this chemistry is outside the committee’s purview. The density of the electrons is expected to decrease if and when aerosols charge negatively. Thus aerosol charging may be responsible for large drops in electron density observed by ground-based radars, However, contrary to expectations, in situ rocket measurements often find positively charged aerosols. It is clear, there- fore, that aerosol charging mechanisms are not yet understood. Charging models are needed that include the effects of collisions between neutrals, electrons, and ions, as well as possible effects related to high aerosol densities. The continuous nucleation and evaporation of the aerosols, their wind-driven transport, and the subsequent buildup of electric fields due to possible charge separation must also be investigated. Clearly, this region offers a rich set of basic physical phenomena that at the moment escape our full understanding. Progress requires a combination of in situ and laboratory experiments, as well as the development of theoretical models. In weakly ionized plasmas such as the mesosphere, ion-neutral collisions can- not be neglected. The interpretation of Langmuir probe measurements, our most basic plasma diagnostics tool, remains difficult in this environment due to the ab- sence of detailed theoretical models. A rocket transitions from a collisional regime at low altitude, where fluid formalism can be used, to a regime where the collisional mean free path becomes larger than a rocket (at around 80 km in altitude) and the physics is best described using a kinetic approach. Models that connect these regimes smoothly do not yet exist. Plasmas can also interact with radiation fields such as in stellar atmospheres. While understanding of radiative transfer in dynamic gaseous media is relatively well developed, the importance of the interactions between electromagnetic radia- tion and matter in the plasma state has only recently been recognized. Understand- ing these interactions can provide insights into radiation-plasma coupling in the other astrophysical systems. CONCLuSIONS AND RECOMMENDATIONS FOR THIS TOPIC It is clear from the examples presented in the preceding section that progress on the broad goal of understanding the universe and on many of the central ques- tions in space physics and astrophysics is dependent on a better understanding of plasma phenomena. As an indication of the importance of plasma science to space and astrophysics, note that many of the highly recommended ground-based and

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Plasma science 10 TABLE 5.1 Astrophysics and Space-Physics Projects Illustrating the Overlap Between NASA Missions and Plasma Physics Initiative Plasma Interest Astrophysics Advanced Solar Telescope (AST) Magnetic fields, solar flares, dynamos Constellation-X Observatory (Con-X) Black holes, x-ray clusters Gamma-ray Large Area Space Telescope (GLAST) Particle acceleration, compact objects Very Energetic Radiation Imaging Telescope Array Cosmic rays, particle acceleration System (VERITAS) Solar Dynamics Observatory (SDO) Solar magnetic field, space weather Square Kilometer Array (SKA) Early universe, compact objects Energetic X-ray Imaging Survey (EXIST) Telescope Black holes, the transient x-ray sky Frequency Agile Solar Radio Telescope (FASR) Solar corona, solar flares, space weather Advanced Radio Interferometry Between Space Acceleration and collimation of jets and Earth (ARISE) James Webb Space Telesope (JWST) Star and planet formation, neutral-plasma interactions Combined Array for Research in Millimeter-wave ISM, neutral-plasma interactions Astronomy (CARMA) Space Advanced Composition Explorer (ACE) Solar wind monitor Cluster Multipoint studies of plasma boundaries Reuven Ramaty High Energy Solar Spectroscopy Advanced imaging of solar plasma processes Imager (RHESSI) Fast Auroral Snapshot Explorer (FAST) Auroral plasma processes Wind satellite Solar wind plasmas Rockets/balloons Ionosphere and mesospheric studies Solar Terrestrial Relations Observatory (STEREO) Stereo imaging of solar processes Solar-B, Hinode Solar imaging Time History of Events and Macroscale Global reconfiguration of Earth’s magnetosphere; Interactions during Substorms (THEMIS) study of the magnetism and instabilities of the Sun Solar Dynamics Observatory Solar magnetic fields, dynamo, variability Interstellar Boundary Explorer (IBEX) Exploring boundary with ISM Magnetospheric Multiscale (MMS) Multiple-point plasma processes Polar Auroral processes Radiation Belt Storm Probe (RBSP) Radiation belt studies Juno Jupiter’s magnetosphere and aurora NOTE: The first half of the table shows some astrophysical missions recommended in Astronomy and Astrophysics in the New Millennium and their connection to plasma physics. The second half of the table shows some space physics missions recommended in The Sun to the Earth—and Beyond as well as some currently operating missions and their connections to plasma physics. space-based initiatives of the National Research Council’s 2001 decadal survey of astronomy and astrophysics1 are intimately related to the plasma science contained in this report. Table 5.1 lists these major and moderate-scale initiatives along with 1 NRC, Astronomy and Astrophysics in the New Millennium, Washington, D.C.: National Academy Press, 2001.

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sPace astroPhysical Plasmas 11 and the plasma physics that is addressed by each. Interpreting observations from many of the new frontiers in experimental astrophysics—such as large-area neutrino telescopes (e.g., IceCube) and perhaps even gravitational-wave observatories (e.g., LIGO and LISA)—will require understanding the plasma physics of the underly- ing astrophysical sources. Table 5.1 also lists ongoing and upcoming space, solar, and heliospheric missions that are reliant on plasma physics to address both their underlying science goals and their exploration mission objectives; the list of initia- tives is largely based on the National Research Council’s 2003 decadal survey of solar and space physics.2 Conclusion: Plasma physics is increasingly important for research in space physics and astrophysics. Also, space physics and astrophysics are providing critical insights that illuminate fundamental aspects of plasmas. Indeed, some compelling research questions in plasmas physics will be best answered by research in space and astrophysical contexts. This chapter presents examples of where space and astrophysical observations have led to new a understanding of basic plasma physics processes, including fast reconnection, dusty plasma interactions, and high-energy particle acceleration. The corollary to using plasma physics to explore space is that space and astrophysical plasma physics are opening up many new regimes of plasma physics (e.g., general relativistic plasmas) that have not been and cannot be studied in laboratory set- tings. Many frontiers remain to be explored, such as plasma physics on cosmologi- cal scales. New missions and telescopes will continue to add to the plasma physics that can be studied. Deployment of new measurement techniques, such as using networks of sensors to develop near-real-time multipoint measurements of mac- roscopic plasma phenomena, also promises to offer a watershed opportunity. Conclusion: given the growing role of plasma physics in space science and astrophysics, it is essential that undergraduate and graduate physics and astronomy curricula include some fluid mechanics, magnetohydrodynam- ics, and plasma physics as a basic requirement. It is uncommon for undergraduate physics and astronomy curricula to include any fluid mechanics, MHD, or plasma physics. These subjects are also missing from many graduate astronomy curricula. Thus many Ph.D. candidates in space and astrophysics are poorly prepared to meet the many challenges and opportunities in plasma-related space physics and astrophysics. 2 NRC, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, Washington, D.C.: The National Academies Press, 2003.

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Plasma science 12 Conclusion: Progress in understanding the fundamental plasma processes in many space and astrophysical phenomena is greatly leveraged by close com- munication among space, astrophysical, and laboratory plasma scientists. The diversity of regimes studied in space physics and astrophysics makes it important to highlight the connections between the different plasma regimes stud- ied in space and astrophysics and the related fields of laboratory plasma physics described in this report. There are many examples of such connections in addition to those discussed in the text. For example, laboratory studies of the equations of state and opacity of dense matter are a crucial ingredient used in models of dense astrophysical plasmas. As another example, electromagnetic wave–plasma inter- actions and related phenomena in the upper atmosphere have close analogies to terrestrial technologies. Dusty plasmas, which were first observed and studied in space, have been the topic of intense study in laboratory experiments. In addition, the physics of dusty plasmas is crucial for understanding the plasma nucleation of nanocrystals for photonics and for preventing particle contamination of silicon wafers during plasma processing for microelectronics fabrication. The fundamental plasma–particle interactions occurring in Earth’s mesosphere are directly analo- gous to those occurring in laboratory plasmas. In a number of research areas, the collaboration between the laboratory, space, and astrophysical communities has led to significant scientific progress.3 Studies of common plasma processes—rather than the large-scale morphology of observed systems—provide the most promising linkages for the different plasma physics communities. The six key plasma processes and questions discussed in Chapter 1 define the linking processes in a broader sense. To isolate a process it is critical to ask one of the three pervasive technical questions in this chapter. To what extent is the plasma science independent of the regime? Where the science is regime- independent, collaboration can effectively leverage individual community efforts. Maintaining and strengthening the linkages between communities is therefore highly desirable. Recommendation: Agency coordination mechanisms such as the Physics of the universe Interagency Working group and the Astronomy and Astro- physics Advisory Committee should explicitly include plasma physics when they coordinate research in laboratory, space, and astrophysical plasma sci- ence. Such coordination would be greatly facilitated by improved steward- ship of laboratory plasma science by DOE’s Office of Science. 3 For more information on the connections between laboratory HED experiments and astrophysics, please see The X-Games Report.

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sPace astroPhysical Plasmas 1 and NASA and NSF support most of the studies of plasmas phenomena in space and astrophysics. Studies of fundamental plasma processes in laboratory plasma science are supported by DOE (in NNSA and OFES) and, to a lesser extent, by NSF. For instance, readers will note that research on magnetic reconnection is taking place under NASA’s auspices as part of space plasma physics, under NSF and DOE’s auspices with basic laboratory experiments, and even under the auspices of DOE’s magnetic fusion research program as it studies self-organization in toroidal plas- mas. The separation of funding sources could impede effective strategies to attack key plasma problems simultaneously from several angles. Such a multipronged attack cannot be achieved without close collaboration between scientists and agen- cies of the federal government in all communities. On the other hand it would not be desirable to separate space physics and astrophysics plasma research from their broader context in space and astrophysics. Although the committee was not charged with conducting a comprehensive review of the federal solar and space physics research portfolio, it is important to note that the above recommendation has significant overlap with the recommen- dations of NRC’s Solar and Space Physics Survey Committee for its 2003 report The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics.4 In other words, the traditional space and astrophysics communities and the traditional plasma science community have identified enhanced federal coor- dination as a key action item. 4 NRC, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, Washington, D.C.: The National Academies Press, 2003, p. 12.