Microphysics in Astrophysical Plasmas
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Microphysics in Astrophysical Plasmas Steven J. Schwartz · Ellen G. Zweibel · Martin Goldman
Received: 25 January 2013 / Accepted: 8 March 2013 © The Author(s) 2013. This article is published with open access at Springerlink.com
Abstract Although macroscale features dominate astrophysical images and energetics, the physics is controlled through microscale transport processes (conduction, diffusion) that mediate the flow of mass, momentum, energy, and charge. These microphysical processes manifest themselves in key (all) boundary layers and also operate within the body of the plasma. Crucially, most plasmas of interest are rarefied to the extent that classical particle collision length- and time-scales are long. Collective plasma kinetic phenomena then serve to scatter or otherwise modify the particle distribution functions and in so-doing govern the transport at the microscale level. Thus collisionless plasmas are capable of supporting thin shocks, current sheets which may be prone to magnetic reconnection, and the dissipation of turbulence cascades at kinetic scales. This paper lays the foundation for the accompanying collection that explores the current state of knowledge in this subject. The richness of plasma kinetic phenomena brings with it a rich diversity of microphysics that does not always, if ever, simply mimic classical collision-dominated transport. This can couple the macro- and microscale physics in profound ways, and in ways which thus depend on the astrophysical context. Keywords Microphysics · Plasmas · Astrophysics · Space plasmas
S.J. Schwartz () Blackett Laboratory, Imperial College London, South Kensington, London SW7 2AZ, UK e-mail: [email protected] E.G. Zweibel Astronomy Department, University of Wisconsin, Madison, WI 53706, USA e-mail: [email protected] M. Goldman Department of Physics, University of Colorado, Boulder, CO 80309-0390, USA e-mail: [email protected]
S.J. Schwartz et al.
1 Introduction The astrophysical world is filled with plasmas, from the solar atmosphere through supernova remnants to distant galaxies. Despite these diverse environments, there are common underlying physical mechanisms at work. Shock waves form at flow interaction regions, current layers breakdown to release bottled-up magnetic energy, and turbulence tangles magnetic fields and cascades energy to small scales where it is dissipated. In the classical view, these and many more phenomena are controlled by transport processes (diffusion, conduction, etc.) that are mediated by inter-particle collisions. The resulting collision frequencies and transport coefficients are then used in a fluid approach to the physics. In such an approach, these coefficients depend only on the local state parameters (e.g., density, temperature) independent of the large-scale region of interest. In this view, the media never stray far from Maxwellian thermodynamic equilibrium. If we lived in a fluid Universe, there would be no solar flares, no ultra-relativistic cosmic rays, and no Aurora Borealis. However
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