The Heating of Test Particles in Numerical Simulations of Alfvenic Turbulence

TL;DR

This study investigates how charged particles are heated in 3D MHD turbulence simulations, revealing different heating mechanisms depending on particle gyrofrequency and showing that turbulence can cause significant parallel heating, with implications for solar wind and astrophysics.

Contribution

The paper provides new insights into particle heating mechanisms in Alfvenic turbulence, highlighting the role of resonance broadening and the impact of turbulence scales on heating efficiency.

Findings

Particles with gyrofrequency near turbulent fluctuation frequency undergo strong perpendicular heating.

Particles with large gyrofrequency experience strong parallel heating.

Turbulence produces significant parallel heating but limited perpendicular heating above proton Larmor radius.

Abstract

We study the heating of charged test particles in three-dimensional numerical simulations of weakly compressible magnetohydrodynamic (MHD) turbulence (``Alfvenic turbulence''); these results are relevant to particle heating and acceleration in the solar wind, solar flares, accretion disks onto black holes, and other astrophysics and heliospheric environments. The physics of particle heating depends on whether the gyrofrequency of a particle is comparable to the frequency of a turbulent fluctuation that is resolved on the computational domain. Particles with these frequencies nearly equal undergo strong perpendicular heating (relative to the local magnetic field) and pitch angle scattering. By contrast, particles with large gyrofrequency undergo strong parallel heating. Simulations with a finite resistivity produce additional parallel heating due to parallel electric fields in small-scale current sheets. Many of our results are consistent with linear theory predictions for the particle heating produced by the Alfven and slow magnetosonic waves that make up Alfvenic turbulence. However, in contrast to linear theory predictions, energy exchange is not dominated by discrete resonances between particles and waves; instead, the resonances are substantially ``broadened.'' We discuss the implications of our results for solar and astrophysics problems, in particular the thermodynamics of the near-Earth solar wind. We conclude that Alfvenic turbulence produces significant parallel heating via the interaction between particles and magnetic field compressions (``slow waves''). However, on scales above the proton Larmor radius, Alfvenic turbulence does not produce significant perpendicular heating of protons or minor ions.