Particle Acceleration by MHD Turbulence

We discuss particle acceleration by strong and weak MHD turbulence in magnetic pressure (low-beta) and gaseous pressure (high-beta) dominated plasmas. We consider the acceleration by large scale compressions in both slow and fast particle diffusion limits. We compare the results with the acceleration rate that arises from resonance scattering and Transit-Time Damping (TTD). We establish that fast modes accelerate particles more efficiently than slow modes. We find that particle acceleration by pitch-angle scattering and TTD dominates acceleration by slow or fast modes when the spatial diffusion rate is small. When the rate of spatial diffusion of particles is high, we establish a substantial enhancement of the efficiency of particle acceleration by slow modes in weak turbulence. We show that highly supersonic turbulence is an efficient agent for particle acceleration. We find that even incompressible turbulence can accelerate particles on the scales comparable with the particle mean free path