Filamentary Diffusion of Cosmic Rays on Small Scales

TL;DR

This study explores how cosmic rays diffuse anisotropically near their sources within turbulent magnetic fields, leading to filamentary structures and irregular photon emission patterns, with a transition to standard diffusion after about 10,000 years.

Contribution

It provides a detailed analysis of the anisotropic diffusion of cosmic rays close to sources and characterizes the transition to isotropic diffusion, including the resulting photon emission features.

Findings

CRs diffuse anisotropically near sources, forming filamentary structures

Transition to standard diffusion occurs around 10^4 years, depending on parameters

Photon emission patterns reflect the irregular CR distributions

Abstract

We investigate the diffusion of cosmic rays (CR) close to their sources. Propagating individual CRs in purely isotropic turbulent magnetic fields with maximal scale of spatial variations Lmax, we find that CRs diffuse anisotropically at distances r <~ Lmax from their sources. As a result, the CR densities around the sources are strongly irregular and show filamentary structures. We determine the transition time t* to standard diffusion as t* ~ 10^4 yr (Lmax/150 pc)^b (E/PeV)^(-g) (Brms/4 muG)^g, with b ~ 2 and g = 0.25-0.5 for a turbulent field with Kolmogorov power spectrum. We calculate the photon emission due to CR interactions with gas and the resulting irregular source images.