Abstract
Among the various acceleration mechanisms which have beensuggested as responsible for the nonthermal particle spectra andassociated radiation observed in many astrophysical and spacephysics environments, diffusive shock acceleration appears to bethe most successful. We review the current theoreticalunderstanding of this process, from the basic ideas of how ashock energizes a few reactionless particles to the advancednonlinear approaches treating the shock and acceleratedparticles as a symbiotic self-organizing system. By means ofdirect solution of the nonlinear problem we set the limit to thetest-particle approximation and demonstrate the fundamental roleof nonlinearity in shocks of astrophysical size and lifetime. Westudy the bifurcation of this system, proceeding from thehydrodynamic to kinetic description under a realistic conditionof Bohm diffusivity. We emphasize the importance of collectiveplasma phenomena for the global flow structure and accelerationefficiency by considering the injection process, an initialstage of acceleration and, the related aspects of the physics ofcollisionless shocks. We calculate the injection rate fordifferent shock parameters and different species. This, togetherwith differential acceleration resulting from nonlinearlarge-scale modification, determines the chemical composition ofaccelerated particles. The review concentrates on theoreticaland analytical aspects but our strategic goal is to link thefundamental theoretical ideas with the rapidly growing wealth ofobservational data.
Published Version
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