Regimes of cosmic-ray diffusion in Galactic turbulence

TL;DR

This paper investigates cosmic-ray diffusion in Galactic turbulence, emphasizing the importance of wave spectrum boundaries and magnetic field components, and applies the findings to explain cosmic-ray transport in the Milky Way.

Contribution

It provides a quantitative analysis of parallel and perpendicular cosmic-ray diffusion coefficients considering wave spectrum boundaries and turbulence levels, with applications to Galactic cosmic-ray transport.

Findings

Purely diffusive models conflict with inner Galaxy observations.

Galactic wind inclusion aligns models with data in the central 5 kpc.

Outer Galaxy diffusion is dominated by perpendicular diffusion, shifting to parallel at larger radii.

Abstract

Cosmic-ray transport in astrophysical environments is often dominated by the diffusion of particles in a magnetic field composed of both a turbulent and a mean component. This process, which is two-fold turbulent mixing in that the particle motion is stochastic with respect to the field lines, needs to be understood in order to properly model cosmic-ray signatures. One of the most important aspects in the modeling of cosmic-ray diffusion is that fully resonant scattering, the most effective such process, is only possible if the wave spectrum covers the entire range of propagation angles. By taking the wave spectrum boundaries into account, we quantify cosmic-ray diffusion parallel and perpendicular to the guide field direction at turbulence levels above 5% of the total magnetic field. We apply our results of the parallel and perpendicular diffusion coefficient to the Milky Way. We show that simple purely diffusive transport is in conflict with observations of the inner Galaxy, but that just by taking a Galactic wind into account, data can be matched in the central 5 kpc zone. Further comparison shows that the outer Galaxy at $>5\,$kpc, on the other hand, should be dominated by perpendicular diffusion, likely changing to parallel diffusion at the outermost radii of the Milky Way.