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.
Highlights
The origin of cosmic rays has been the subject of much research since the first detection of cosmic rays in 1912, see e.g. [1] for a review
We find that a power-law fit performs well for the parallel and perpendicular components of the diffusion coefficient, i.e. i ∝ E i, with i = ∥, ⟂
We find that these spectral indices are functions of the turbulence level, i = i(b∕B)
Summary
The origin of cosmic rays has been the subject of much research since the first detection of cosmic rays in 1912, see e.g. [1] for a review. In case of diffusion-dominance, the energy spectrum of cosmic rays after propagation is steepened, i.e. n(E) ∝ E− s− i [23] These arguments are based on QLT, in which only linear terms in the distortions of the electromagnetic fields and particle population with respect to the undisturbed fields are being considered to simplify the kinetic equations and to facilitate their analytical treatment. Using simulations in the fully resonant scattering regime, we quantitatively investigate the energy behavior of the diffusion coefficient as a function of the turbulence ratio b/B
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