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Plasma Science: From Fundamental Research to Technological Applications 7 Plasma Astrophysics Plasma physics is relevant to almost every area of astrophysics, from magnetized, highly conducting stellar and interstellar plasma to gravitationally interacting many-body systems such as star clusters and galaxies. In some cases, the plasma physics is quite standard and requires only the application of known results. In other cases, the problem lies beyond the current frontiers of knowledge. Yet, plasma physics is not part of the standard graduate astrophysics curriculum, and plasma astrophysics has no distinct home at any federal funding agency. This chapter briefly describes some recent accomplishments and outstanding problems in plasma astrophysics, as well as education in and funding of plasma astrophysics. RECENT ACCOMPLISHMENTS IN PLASMA ASTROPHYSICS Any list of recent accomplishments is bound to be incomplete, but the work discussed below is representative. Magnetized Disks, Winds, and Jets Astrophysical interest in this problem goes back at least as far as the 1950s, when Fred Hoyle speculated that the early Sun could have transferred angular momentum to the protoplanetary disk via magnetic torques. The first quantitative theories began with the solar wind, which is observed to be magnetized. Simple models were developed to show that magnetic torques exerted by the solar wind could have removed significant quantities of angular momentum from
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Plasma Science: From Fundamental Research to Technological Applications the Sun and, by implication, that magnetized winds could play an important role in spinning down stars. Later, spurred by observations of accretion disks and jets around a wide variety of objects including protostars, white dwarfs, neutron stars, and black holes, astrophysicists developed models of magnetized winds and jets in disk geometry, included relativistic effects, strong magnetic fields, rapid rotation, and the effects of MHD waves and instabilities on the disks and the outflows. (See Figure 7.1.) Particle Acceleration in Shocks Although our understanding of high-Mach-number shocks is seriously incomplete, studies of particle acceleration in shocks have given us the best theories to date of cosmic-ray acceleration in the interstellar medium. The most notable successes of the theory are that it predicts approximately the correct power-law index of the energy spectrum, cosmic-ray intensity, and cosmic-ray composition (with the exception of the electron-to-ion ratio). Progress has been made on the analytical front through both kinetic and hydrodynamical descriptions of the particles and the shock and on the computational front through Monte Carlo simulations. Magnetized Convection in Stars The subject of stellar convection has a long history, since it was recognized many years ago that the radiative energy flux through a stellar envelope is limited by convective instability. Interest in the interaction of magnetic fields with convection stems from observations of the solar magnetic activity cycle and similar cycles on other stars, which show that magnetic fields are rapidly regenerated and reconfigured in the interiors of convective stars. Until recently, stellar convection was described only by dimensional arguments or mixing length theory. With the development of parallel and massively parallel computer architecture, it has become possible to simulate compressible convection in three dimensions and to include the effects of magnetic fields. Although the smallest relevant length scales are still unresolved by these calculations, the effects of buoyancy, concentration of flux into ropes, and dynamo activity—all processes that are believed to play an important role in the dynamics of stellar magnetic fields—are observed and can be studied. Formation of Low-Mass Stars It was recognized long ago that the ratio of magnetic flux to mass is much higher in the interstellar medium than it is in stars. It was proposed that interstellar clouds are supported against their gravitational fields by magnetic forces, that the fields slowly escape from the clouds by ion-neutral relative drift, and that the
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Plasma Science: From Fundamental Research to Technological Applications FIGURE 7.1 A plasma kinetic-theory model of a relativistic shock wave in the Crab Nebula. Upper left: Contour plot of the surface brightness of x-ray emission at 0.8 Å. The ''wisp" features are thought to be visible manifestations of the otherwise radiationless outflow of rotational energy from the central pulsar. Upper right: Geometry of the outflow from the pulsar used in the construction of the theoretical model. The pulsar is assumed to lose energy in the form of a magnetohydrodynamic wind, flowing relativistically in an angular sector around the rotational equator of the pulsar. The magnetic field direction is orthogonal to the radial flow. The wind's composition is a mixture of electrons, positrons, and heavy ions, and it is quasi-neutral in the region upstream of the shock wave that terminates the outflow. Estimates indicate that a shock wave forms in the region of the observed wisps. The vector n points toward the observer. Lower panel: Comparison of the surface brightness (solid line) measured in the strip between the dashed lines in the upper panel with that predicted by the model (dashed line). The model represents the electron-positron pairs as a relativistically hot Maxwellian fluid, heated by the collisionless subshock at the leading edge of the shock structure. Heavy ions are modeled as a stream of particles gyrating in the electromagnetic field of the shock, compressing the magnetic field and pair plasma at each turning point of the ions' orbit. Each such compression appears as a surface brightness enhancement. The model successfully predicts the brightness of the faint wisp at -7 arc sec. (Reprinted, by permission, from M. Hoshino, J. Arons, Y.A. Gallant, and A.B. Langdon, Astrophysical Journal 390:454, 1992, and Y.A. Gallant and J. Arons, Astrophysical Journal 435:230, 1994. Copyright © 1992, 1994 by the American Astronomical Society.)
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Plasma Science: From Fundamental Research to Technological Applications clouds collapse and form stars once a sufficient amount of their magnetic flux has been removed. This picture has been confirmed and extended by extensive theoretical calculations, including static models of magnetized, self-gravitating clouds; dynamical models of gravitational collapse; and both analytical and numerical calculations of ion-neutral drift. PROBLEMS IN PLASMA ASTROPHYSICS The panel has not attempted to make an exhaustive list of problems in plasma astrophysics, but instead has chosen a few problems that arise in a broad variety of physical environments, illustrating how plasma physics touches almost every part of astrophysics. Some of these problems are in the realm of space plasma physics as well. Dense Stellar Plasmas The mass density and temperature at the center of the Sun, which is an ordinary, low-mass star, are predicted to be about 100 g/cm3 and 2 × 107 K, respectively. The energy produced by nuclear reactions diffuses outward as radiative energy, with most of the opacity due to bound-free transitions in elements heavier than helium. At these temperatures and densities, atoms are significantly perturbed by their nearest neighbors. Recent attempts to take these many-body effects into account when calculating the opacity and equation of state of dense stellar material have produced strikingly different results from earlier calculations, which has injected substantial uncertainty into models of solar-type stars and their evolution. This problem is at the intersection of plasma physics, statistical mechanics, and atomic physics. Thermal Conduction in Plasmas Observations suggest sharp temperature interfaces between the solar corona and lower atmosphere and at the boundaries of interstellar clouds. These interfaces are sharp in the sense that the inferred temperature scale height is comparable to the electron mean free path. The transport of heat becomes strongly nonlocal, and the electron distribution function becomes non-Maxwellian. Attempts to solve this problem have ranged from the application of theories of saturated heat flux regulated by ion-acoustic instabilities to attempts at full kinetic theory solutions of the Boltzmann equation. Structure of Collisionless Shocks Astrophysical shock waves are produced by energetic, impulsive events ranging from solar and stellar flares to sequential supernova explosions in asso-
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Plasma Science: From Fundamental Research to Technological Applications ciations of massive stars. Because mean free paths are long, these shocks must be collisionless. Remote sensing by spectroscopy shows that electrons as well as ions are heated to high temperatures. How is the ion distribution thermalized? How is this energy fed into the electrons? How is a small tail of particles accelerated to high energies, as is observed in the interplanetary medium? What is the back-reaction of the accelerated particles on the shock? These remain outstanding problems, because the Mach numbers are so high that the shocks are probably turbulent. Numerical simulations appear to be the most promising way to attack the problem at this point. Acceleration of Particles to High Energies Spiral galaxies appear to be permeated by a component of energetic particles, cosmic rays. In our galaxy the distribution function can be followed from subrelativistic energies to energies as high as 1021 eV. The most energetic particles cannot be confined by the galactic magnetic field. The energy density of these cosmic rays is similar to both the magnetic and the turbulent energy density in the galactic disk. How are these particles accelerated, and how do they propagate through the galaxy? The prevailing theories have particles at energies less than about 1015 eV accelerated by the Fermi mechanism in shocks and predict that they will be trapped within the galaxy by resonant scattering off Alfvén waves excited by their own anisotropy. For more energetic particles, the confinement is problematic, and the origin may be extragalactic. Hydromagnetic Turbulence There is abundant evidence for hydromagnetic turbulence in objects as diverse as stellar convection zones, the interstellar gas in galaxies, and the gas in clusters of galaxies. Turbulence can provide hydrodynamic forces (e.g., pressure support in interstellar clouds or acceleration in stellar winds), can lead to transport coefficients such as viscosity or resistivity that are much larger than their molecular values, and can provide significant heating through dissipation. Yet, we do not have a complete theory of hydromagnetic turbulence, and simulations, which are of great educational value, do not yet resolve the full range of relevant scales. Progress in understanding MHD turbulence will probably be made through a combination of direct observation (such as in situ measurements in the solar wind), simulations, and analytical theory. Magnetic Reconnection The magnetic Reynolds number (or Lundquist number) of astrophysical plasmas is typically huge, ranging from 108 in the solar interior to 1021 in the galactic interstellar medium. The naive conclusion is then that magnetic flux is perma-
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Plasma Science: From Fundamental Research to Technological Applications nently frozen into the plasma and that the field never changes topology. Yet, magnetic fields apparently do change topology (e.g., the star formation process seems to reconnect the magnetic field), and there is strong evidence that magnetic reconnection is an important source of energy in solar flares. How do field lines reconnect at very high magnetic Reynolds number? Present thinking suggests a two-stage process: some ideal magnetohydrodynamic effect creates steep gradients; then reconnection proceeds. We need a more fundamental understanding of the reconnection process itself; many of the fusion-oriented simulations have inappropriate boundary conditions for astrophysical systems. We also need a better understanding of the "ideal" current concentration phase. The Magnetization of the Universe Stars, galaxies, and the gas in clusters of galaxies possess magnetic fields. Standard cosmology predicts that the big bang did not produce a magnetic field. How and when did the universe become magnetized? Did large-scale, intergalactic fields form first and become incorporated into smaller structures, or did fields form first in stars, which then seeded their ambient medium through winds and supernova explosions? Are astrophysical magnetic fields nearly permanent, as suggested by their very long ohmic decay times, or are they constantly destroyed, regenerated, and reconfigured by turbulent dynamos? Laboratory Experiments There have been few laboratory experiments dedicated to plasma astrophysics, and any such experiments must carefully scale properly from the laboratory to the real astrophysical system. Areas in which experiments could be helpful include MHD turbulence, magnetic reconnection, shock waves, particle acceleration, dusty plasmas, and heat conduction. The status and future promise of laboratory experiments in many of these and related areas are discussed in Chapter 8. TRAINING IN PLASMA ASTROPHYSICS How do graduate students become equipped to deal with problems in plasma astrophysics? The standard graduate curriculum in astrophysics contains graduate physics courses, such as quantum mechanics, electrodynamics, statistical mechanics, classical mechanics—more or fewer, depending on the school and the inclination of the student. Then there are standard astrophysics courses, such as stellar structure and evolution, stellar atmospheres and radiative transfer, interstellar medium, and galaxies and cosmology. At many universities, no courses in plasma physics are taught in the physics or astrophysics departments. Such courses may be given in an engineering or applied science department, but these
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Plasma Science: From Fundamental Research to Technological Applications often have too technological an orientation to attract astrophysics students. Some plasma physics may or may not be integrated into one or more of the astrophysics courses, depending on the inclination of the instructor. Therefore, very few astrophysics students receive much formal exposure to plasma physics, and many astrophysicists view it as an arcane specialty. Many astrophysicists would like to learn more plasma physics when motivated to do so by developments in their subject. For example, recent measurements of magnetic field strengths in dense, star-forming interstellar clouds have shown that the fields are large enough to strongly affect or even dominate the dynamics. This has spawned a real interest in MHD among interstellar medium researchers, and a number of people who ignored magnetic fields throughout most of their careers are now writing papers on them. Such people would benefit from a good, modern text on plasma physics, one not oriented toward fusion plasmas, but stressing astrophysically interesting applications and using astrophysically relevant parameters and boundary conditions. Such a book could consist of chapters contributed by experts, provided that a good editor and refereeing system kept the quality high. Such a book could also be used for a graduate course or seminar. FUNDING FOR PLASMA ASTROPHYSICS Most plasma astrophysics by individual investigators is funded through the NSF and NASA. Some solar and space plasma physics has been funded by the Air Force and the Office of Naval Research, and some DOE funding has arrived through support for national centers. Plasma astrophysics funding at the NSF suffers from a problem common to all of theoretical astrophysics: programs are organized by wavelength band or class or object, rather than by physical process. This discourages broad proposals. Yet, one of the exciting aspects of plasma astrophysics is that the same processes are at work under many different astrophysical conditions. Both the 1980 and 1990 NAS-sponsored decadal surveys of astrophysics (the Field1 and Bahcall2 Committees, respectively) recommended that the NSF establish a theoretical astrophysics program. With the notable and important exceptions of its Astrophysical Theory and Space Physics Theory programs, NASA tends to support research focused on its missions. This has led to better support for space plasma physics, where much of the data is mission-relevant, than it has for plasma astrophysics, where fund- 1 National Research Council, Astronomy Survey Committee, Astronomy and Astrophysics for the 1980s, National Academy Press, Washington, D.C., 1982. 2 National Research Council, Astronomy and Astrophysics Survey Committee, The Decade of Discovery in Astronomy and Astrophysics, National Academy Press, Washington, D.C., 1991.
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Plasma Science: From Fundamental Research to Technological Applications ing to develop new theories of distant objects is sometimes deemed an unacceptable stretch of the NASA mission. The panel would welcome an expansion of funding for basic research at NASA. SUMMARY Plasma astrophysics is an exciting and important area of astrophysics that is relevant to a wide variety of astrophysical phenomena and draws on an equally wide variety of topics in plasma physics. Problems in plasma astrophysics could stimulate important basic research. Yet the potential of plasma astrophysics is underrealized. The field is small and lacks the critical mass to provide graduate training at most institutions, especially given the lack of a suitable textbook. The funding base is diffuse, and plasma astrophysics is not recognized as a branch of astrophysics at any federal funding agency. Both the educational and the funding aspects need to be addressed to bring plasma astrophysics into the mainstream. CONCLUSIONS AND RECOMMENDATIONS Conclusions Plasma astrophysics deals with phenomena and problems that are important to virtually every branch of astronomy and astrophysics. Some of these problems touch on areas that are central to basic plasma physics and have indeed inspired research in basic plasma physics. Yet, plasma astrophysics is not recognized as a coherent discipline by any federal funding agency. Recommendation The panel recommends that interdisciplinary programs be established at the National Aeronautics and Space Administration and the National Science Foundation with the goal of funding research in plasma astrophysics, whether through astronomy, solar-terrestrial research, physics, or computer sciences. The purpose of such programs would be to encourage research in plasma astrophysics, including research on basic processes that are relevant to many astrophysical systems.
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