Energetic Particle Acceleration in Compressible Magnetohydrodynamic Turbulence

TL;DR

This paper investigates how compressible MHD turbulence accelerates energetic particles through different wave modes, revealing the dominant modes under various conditions and the resulting power-law energy distributions.

Contribution

It provides a comprehensive analysis of particle acceleration mechanisms in compressible MHD turbulence, highlighting the roles of Alfvén, slow, and fast modes across different regimes.

Findings

Fast mode dominates particle acceleration in super-Alfvénic turbulence.

Power spectra reveal turbulence inertial range and mode energy ratios.

Particle acceleration results in power-law energy distributions.

Abstract

Magnetohydrodynamic (MHD) turbulence is an important agent of energetic particle acceleration. Focusing on the compressible properties of magnetic turbulence, we adopt test particle method to study the particle acceleration from Alfv\'en, slow and fast modes in four turbulence regimes that may appear in a realistic astrophysical environment. Our studies show that (1) the second-order Fermi mechanism drives the acceleration of particles in the cascade processes of three modes by particle-turbulence interactions, regardless of whether the shock wave appears; (2) not only can the power spectra of maximum acceleration rates reveal the inertial range of compressible turbulence, but also recover the scaling and energy ratio relationship between the modes; (3) fast mode dominates the acceleration of particles, especially in the case of super-Alfv\'enic and supersonic turbulence, slow mode dominates the acceleration for sub-Alfv\'enic turbulence in the very high energy range, and the acceleration of Alfv\'en mode is significant at the early stage of the acceleration; (4) particle acceleration from three modes results in a power-law distribution in the certain range of evolution time. From the perspective of particle-wave mode interaction, this paper promotes the understanding for both the properties of turbulence and the behavior of particle acceleration, which will help insight into astrophysical processes involved in MHD turbulence.