Diffusion Processes in Turbulent Magnetic Fields

TL;DR

This paper investigates how turbulence affects diffusion processes in magnetized media, highlighting the roles of turbulence intensity, Alfven speed, and electron mean free path in heat transfer and cosmic ray diffusion.

Contribution

It introduces the scale $l_A$ as a key parameter and generalizes the effects of turbulence on various diffusion processes, including heat transfer and cosmic ray propagation.

Findings

Heat transfer is suppressed when injection velocity is below Alfven speed.

Turbulence can enhance effective diffusivity through advective transport.

Turbulence is dominant for heat transfer in galaxy clusters.

Abstract

We study of the effect of turbulence on diffusion processes within magnetized medium. While we exemplify our treatment with heat transfer processes, our results are quite general and are applicable to different processes, e.g. diffusion of heavy elements. Our treatment is also applicable to describing the diffusion of cosmic rays arising from magnetic field wandering. In particular, we find that when the energy injection velocity is smaller than the Alfven speed the heat transfer is partially suppressed, while in the opposite regime the effects of turbulence depend on the intensity of driving. In fact, the scale $l_A$ at which the turbulent velocity is equal the Alfven velocity is a new important parameter. When the electron mean free path $\lambda$ is larger than $l_A$, the stronger the the turbulence, the lower thermal conductivity by electrons is. The turbulent motions, however, induces their own advective transport, that can provide effective diffusivity. For clusters of galaxies, we find that the turbulence is the most important agent for heat transfer. We also show that the domain of applicability of the subdiffusion concept is rather limited.