Acceleration via Random Impulses

Fermi-I acceleration can be thought of as a 1-D compression of a magnetized plasma. It heats only in the E// direction. Eventually the pitchangle gets too small to be reflected by the upstream waves, and it escapes. If pitchangle diffusion occurs, a particle may convert E//E_perp, and continue to gain energy.
The waves, or compressive impulses need not be coherent or particularly large, especially if scattering is occurring. Thus the entropy of the energy source is relatively high, the probability is high = high efficiency acceleration. Observations support this.

The waves, or compressive impulses need not be coherent or particularly large, especially if scattering is occurring. Thus the entropy of the energy source is relatively high, the probability is high = high efficiency acceleration. Observations support this.