Revisiting Acceleration of Charged Grains in Magnetohydrodynamic Turbulence

TL;DR

This paper investigates how charged grains are accelerated in magnetohydrodynamic turbulence in the interstellar medium, comparing nonlinear and quasi-linear theories, and exploring the roles of gyroresonance, transit time damping, and Alfven waves.

Contribution

It introduces a nonlinear theory for grain acceleration, showing that guiding center fluctuations slightly reduce velocities and that transit time damping dominates for large grains.

Findings

Guiding center fluctuations reduce grain velocities by less than 15%.

Large grains can reach super-Alfvenic velocities via gyroresonance.

Transit time damping significantly enhances acceleration of large grains.

Abstract

We study the acceleration of charged grains by magnetohydrodynamics (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 with results obtained using quasi-linear theory (QLT). We find for the parameters applicable to the typical ISM phases that the fluctuations of grain guiding center reduce grain velocities by less than 15 percent, 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 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.