Cosmic Ray Small Scale Anisotropies in Slab Turbulence

TL;DR

This paper investigates small-scale anisotropies in cosmic ray arrival directions, proposing a new analytical model that explains observed deviations from isotropy through correlations in turbulent magnetic fields.

Contribution

It introduces a novel analytical approach accounting for flux correlations, explaining small-scale anisotropies in cosmic ray propagation beyond standard diffusion models.

Findings

Analytical model matches numerical simulations of anisotropies.

Higher multipoles are naturally expected from turbulent magnetic fields.

Explains small-scale anisotropies observed by high-statistics observatories.

Abstract

In the standard picture of cosmic ray transport the propagation of charged cosmic rays through turbulent magnetic fields is described as a random walk with cosmic rays scattering on magnetic field turbulence. This is in good agreement with the highly isotropic cosmic ray arrival directions as this diffusion process effectively isotropizes the cosmic ray distribution. High-statistics observatories like IceCube and HAWC have however observed significant deviations from isotropy down to very small angular scales. This is in strong tension with this standard picture of cosmic ray propagation. While large scale multipoles arise naturally, for example due to the earth's motion relative to the isotropic cosmic ray distribution, there is no intuitive mechanism to account for the observed anisotropies at smaller angular scales. By relaxing one of the standard assumptions of quasi linear theory and treating correlations between fluxes of cosmic rays from different directions explicitly we show that higher multipoles also are to be expected from particle propagation through turbulent magnetic fields. We present a first analytical calculation of the angular power spectrum assuming a physically motivated model of the magnetic field turbulence and find good agreement with numerical simulations.