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Fickian and non-Fickian diffusion of cosmic rays

Lookup NU author(s): Dr Luiz Felippe Rodrigues, Dr Andrew Snodin, Dr Graeme Sarson, Professor Anvar Shukurov

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This is the authors' accepted manuscript of an article that has been published in its final definitive form by Oxford University Press, 2019.

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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.


Publication metadata

Author(s): Rodrigues LFS, Snodin A, Sarson G, Shukurov A

Publication type: Article

Publication status: Published

Journal: Monthly Notices of the Royal Astronomical Society

Year: 2019

Volume: 487

Issue: 1

Pages: 975-980

Print publication date: 01/07/2019

Online publication date: 16/05/2019

Acceptance date: 13/05/2019

Date deposited: 28/05/2019

ISSN (print): 0035-8711

ISSN (electronic): 1365-2966

Publisher: Oxford University Press

URL: https://doi.org/10.1093/mnras/stz1354

DOI: 10.1093/mnras/stz1354


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