Physics – Plasma Physics
Scientific paper
Oct 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000aps..dpppi2001m&link_type=abstract
American Physical Society, 42nd Annual Meeting of the APS Division of Plasma Physics combined with the 10th International Congre
Physics
Plasma Physics
Scientific paper
One of the main features of astrophysical shocks is their ability to accelerate particles up to extremely high energies. They, in turn, are proven to be responsible for most of the observed radio, x-ray and gamma radiation from a variety of astrophysical objects. The dominant acceleration mechanism is a Fermi process, also known as the diffusive shock acceleration. Its theory has made a significant evolution from the basic ideas of how a shock energizes a few reactionless particles to the advanced nonlinear approaches treating the shock and accelerated particles as a symbiotic self-organizing system. We present the results of analytical and numerical calculations demonstrating that the acceleration efficiency critically depends on the low-energy particle injection that takes place at a ``truly discontinuous" or ``subshock" part of the entire shock transition which is otherwise greatly smoothed by energetic particles. Furthermore, their essentially inhomogeneous distribution provides free energy for both acoustic and MHD turbulence which, again, regulates the subshock strength and thus the injection rate by heating the upstream plasma. Moreover, the MHD turbulence confines particles to the shock front controlling their maximum energy and bootstrapping acceleration. Therefore, the study of the MHD turbulence in a compressive plasma flow near a shock is a key to understanding of the entire process. The calculation of the injection rate de facto became part of the collisionless shock theory. We stress that the further progress in diffusive shock acceleration theory, needed to meet new standards set by recent revolutionizing development in observational astrophysics (particularly the ground based Cherenkov telescopes and the NASA mission GLAST), is impossible without a significant advance in these two disciplines of plasma physics.
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