model for this was that devised by Fermi (1949). There are several modern variants of his idea; they all involve stochastic processes in which charged particles with effective mean free paths too long to allow them to become thermalized extract free energy via interaction with fluctuations in a dynamic system. The currently most commonly discussed acceleration process, diffusive shock acceleration, is of this type, and I will devote some space to its details momentarily. I note that not all collisionless plasma models depend on Fermi processes, however. In some other settings, such as magnetic reconnection, acceleration is often predicted as a result of locally strong induced electric fields. However, The microphysics of this once again involves the details of a breakdown in thermodynamic or bulk magnetohydrodynamic behaviors (e.g., Lesch & Birk 1998).

Diffusive Particle Acceleration at Shocks
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Starting from the premise above, we can understand so-called diffusive shock acceleration (“DSA”) as a part of the physics of collisionless plasma shocks. Cosmic shocks would mostly not exist if they had to depend on binary interactions; they form, instead due to very complex, nonlinear phenomena involving plasma turbulence (e.g., Quest 1988). The end result is a transition that in some respects 