Energization of charged test particles in magnetohydrodynamic fields: waves vs turbulence picture

TL;DR

This study uses 3D compressible MHD turbulence simulations to investigate how wave modes and coherent structures contribute to particle energization, finding structures play a dominant role over wave resonance.

Contribution

It provides a detailed comparison of linear and non-linear turbulence effects on particle energization and clarifies the main energization mechanism in MHD turbulence.

Findings

Particle energization decreases as linear wave energy increases.

Structures are the main mechanism for particle energization, not wave resonance.

External forcing correlation time affects particle clustering and energization.

Abstract

Direct numerical simulations of 3D compressible MHD turbulence were performed in order to study the relation between waves modes and coherent structures and the consequent energization of test particles. Moreover, the question of which is the main mechanism of this particle energization is rigorously discussed. In particular, using the same initial conditions, we analyzed the non-linear and linear evolution of a turbulent state along with the case of randomized phases. Then, the behavior of the linear and non-linear simulations were compared through the study of time evolution of particle kinetic energy and preferential concentration. Also, spatio temporal spectra were used to identify the presence of wave modes and quantify the fraction of energy around the MHD modes in linear and non-linear simulations. Finally, the variation of the correlation time of the external forcing is studied in detail along with the effect on the particle energization (and clustering) and the presence of wave modes. More specifically, particle energization tends to decrease when the fraction of linear energy increase, supporting the idea that energization by structures is the dominant mechanism for particle energization instead of resonating with wave modes as suggested by Fermi energization theory.