Fickian and non-Fickian diffusion of cosmic rays

TL;DR

This paper compares Fickian and non-Fickian models of cosmic ray diffusion, demonstrating that a telegraph equation approach can more accurately match particle simulations and account for subgrid magnetic field effects.

Contribution

It introduces a non-Fickian diffusion model for cosmic rays using the telegraph equation, improving the accuracy of fluid simulations in complex magnetic fields.

Findings

Non-Fickian model matches test-particle simulations accurately.

The telegraph equation accounts for unresolved magnetic field scales.

Fickian diffusion fails under complex boundary conditions.

Abstract

Fluid approximations to cosmic ray (CR) transport are often preferred to kinetic descriptions in studies of the dynamics of the interstellar medium (ISM) of galaxies, because they allow simpler analytical and numerical treatments. Magnetohydrodynamic (MHD) simulations of the ISM usually incorporate CR dynamics as an advection-diffusion equation for CR energy density, with anisotropic, magnetic field-aligned diffusion with the diffusive flux assumed to obey Fick's law. We compare test-particle and fluid simulations of CRs in a random magnetic field. We demonstrate that a non-Fickian prescription of CR diffusion, which corresponds to the telegraph equation for the CR energy density, can be easily calibrated to match the test particle simulations with great accuracy. In particular, we consider a random magnetic field in the fluid simulation that has a lower spatial resolution than that used in the particle simulation to demonstrate that an appropriate choice of the diffusion tensor can account effectively for the unresolved (subgrid) scales of the magnetic field. We show that the characteristic time which appears in the telegraph equation can be physically interpreted as the time required for the particles to reach a diffusive regime and we stress that the Fickian description of the CR fluid is unable to describe complex boundary or initial conditions for the CR energy flux.