REVISITING ACCELERATION OF CHARGED GRAINS IN MAGNETOHYDRODYNAMIC TURBULENCE

We study the acceleration of charged grains by magnetohydrodynamic (MHD) turbulence in the interstellar medium (ISM). We begin with revisiting gyroresonance acceleration by taking into account the fluctuations of grain guiding center along a uniform magnetic field (i.e., nonlinear theory-NLT). We calculate grain velocities due to gyroresonance by fast MHD modes using the NLT for different phases of the ISM and compare them with results obtained using quasi-linear theory (QLT). We find for the parameters applicable to the typical ISM phases that the fluctuations of the grain guiding center reduce grain velocities by less than 15%, but they can be important for more special circumstances. We confirm that large grains can be accelerated to super-Alfvenic velocities through gyroresonance. For such super-Alfvenic grains, we investigate the effect of further acceleration via transit-time damping (TTD) by fast modes. We find that due to the broadening of the resonance condition in the NLT, the TTD acceleration is not only important for the cosines of grain pitch angle relative to the magnetic field mu > V-A/v, but also for mu < V-A/v where v is the grain velocity and V-A is the Alfven speed. We show that the TTD acceleration is dominant over the gyroresonance for large grains, and can increase substantially grain velocities induced by gyroresonance acceleration. We quantify another stochastic acceleration mechanism arising from low-frequency Alfven waves. We discuss the range of applicability of the mechanisms and their implications.