In sections 4.15 we considered charged particle shock acceleration at the single crossing of a particle through the shock front (see Fig. 4.15.1). This happened when a shock wave propagated through space plasma with homogeneous frozen-in magnetic field. In other, much more frequent, cases, when the propagation of shock wave take place through excited, turbulence space plasma, a particle scatters on magnetic inhomogeneities behind and before the front and has some probability of interacting with the shock front several times (see the illustration of this diffusion process in Fig, 4.21.1).
This type of particle acceleration by shock wave in literature (Krymsky, 1977; Axford et al., 1977; Bell, 1978a,b) is called as regular shock acceleration (because at each crossing of shock front particle gain the energy) or diffusive shock acceleration (because the multi-crossing of shock front is caused by particle scattering and diffusion through magnetic inhomogeneities behind and before the shock front).
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