Turbulent diffusion of streaming cosmic rays in compressible, partially ionized plasma
Abstract
Cosmic rays (CRs) are a dynamically important component of the interstellar medium (ISM) of galaxies. The ~GeV CRs that carry most CR energy and pressure are likely confined by selfgenerated turbulence, leading them to stream along magnetic field lines at the ion Alfvén speed. However, the consequences of selfconfinement for CR propagation on galaxy scales remain highly uncertain. In this paper, we use a large ensemble of magnetohydrodynamical turbulence simulations to quantify how the basic parameters describing ISM turbulence  the sonic Mach number, $\mathcal {M}$ (plasma compressibility), Alfvén Mach number, $\mathcal {M}_{\text{A0}}$ (strength of the largescale field with respect to the turbulence), and ionization fraction by mass, χ  affect the transport of streaming CRs. We show that the largescale transport of CRs whose smallscale motion consists of streaming along field lines is well described as a combination of streaming along the mean field and superdiffusion both along (parallel to) and across (perpendicular to) it; $\mathcal {M}_{\text{A0}}$ drives the level of anisotropy between parallel and perpendicular diffusion and χ modulates the magnitude of the diffusion coefficients, while in our choice of units, $\mathcal {M}$ is unimportant except in the subAlfvénic ($\mathcal {M}_{\text{A0}}\lesssim 0.5$) regime. Our finding that superdiffusion is ubiquitous potentially explains the apparent discrepancy between CR diffusion coefficients inferred from measurements close to individual sources compared to those measured on larger, Galactic scales. Finally, we present empirical fits for the diffusion coefficients as a function of plasma parameters that may be used as subgrid recipes for global ISM, galaxy, or cosmological simulations.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 February 2023
 DOI:
 10.1093/mnras/stac3207
 arXiv:
 arXiv:2205.08174
 Bibcode:
 2023MNRAS.519.1503S
 Keywords:

 magnetohydrodynamics (MHD);
 turbulence;
 methods: numerical;
 (ISM:) cosmic rays;
 Astrophysics  Astrophysics of Galaxies;
 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 Accepted in MNRAS