Cosmic Ray Small-Scale Anisotropies in Quasi-Linear Theory

TL;DR

This paper introduces an analytical method to predict small-scale cosmic ray anisotropies caused by turbulent magnetic fields, aligning well with previous numerical results and enhancing understanding of cosmic ray propagation.

Contribution

It presents the first analytical perturbative approach to compute the angular power spectrum of cosmic ray anisotropies in turbulent magnetic fields.

Findings

Analytical predictions match numerical simulations.

Small-scale anisotropies arise from cosmic ray streaming.

Method applicable to simple isotropic turbulence models.

Abstract

The distribution of arrival directions of cosmic rays is remarkably isotropic, which is a consequence of their repeated scattering in magnetic fields. Yet, high-statistics observatories like IceCube and HAWC have revealed the presence of small-scale structures at levels of 1 part in 10,000 at hundreds of TeV, which are not expected in typical diffusion models of cosmic rays. We follow up on the suggestion that these small-scale anisotropies are a result of cosmic ray streaming in a particular realisation of the turbulent magnetic field within a few scattering lengths in our local Galactic neighbourhood. So far, this hypothesis has been investigated mostly numerically, by tracking test particles through turbulent magnetic fields. For the first time, we present an analytical computation that through a perturbative approach allows predicting the angular power spectrum of cosmic ray arrival directions for a given model of turbulence. We illustrate this method for a simple, isotropic turbulence model and we find remarkable agreement with the results of numerical studies.