Asymmetric diffusion of cosmic rays

TL;DR

This paper demonstrates that high-amplitude magnetohydrodynamic turbulence causes asymmetric diffusion of cosmic rays, significantly affecting their distribution in the galaxy beyond standard diffusion models.

Contribution

It introduces an analytical model showing how turbulence induces asymmetric cosmic ray diffusion, challenging traditional symmetric diffusion assumptions.

Findings

Cosmic ray density differs from standard diffusion predictions.

Asymmetric diffusion impacts cosmic ray distribution in the galaxy.

Limitations of the convection-diffusion equation are highlighted.

Abstract

Cosmic ray propagation is diffusive because of pitch angle scattering by waves. We demonstrate that if the high-amplitude magnetohydrodynamic turbulence with $\tilde B/\langle B\rangle \sim 1$ is present on top of the mean field gradient, the diffusion becomes asymmetric. As an example, we consider the vertical transport of cosmic rays in our Galaxy propagating away from a point-like source. We solve this diffusion problem analytically using a one-dimensional Markov chain analysis. We obtained that the cosmic ray density markedly differs from the standard diffusion prediction and has a sizable effect on their distribution throughout the galaxy. The equation for the continuous limit is also derived, which shows limitations of the convection-diffusion equation.