TeV-PeV Cosmic-Ray Anisotropy and Local Interstellar Turbulence

TL;DR

This paper models the large-scale anisotropy of TeV-PeV cosmic rays using interstellar turbulence theories, revealing that anisotropy shapes can probe turbulence properties and that local turbulence influences small-scale anisotropies.

Contribution

It introduces a new approach to use cosmic-ray anisotropy shapes as probes of interstellar turbulence and presents numerical simulations down to 3 TeV energies.

Findings

Large-scale anisotropy is not a simple dipole shape.

IceTop data fits Goldreich-Sridhar turbulence models.

Local turbulence causes weak non-gyrotropic small-scale anisotropies.

Abstract

We calculate the shape of the large-scale anisotropy of TeV-PeV cosmic-rays (CR) in different models of the interstellar turbulence. In general, the large-scale CR anisotropy (CRA) is not a dipole, and its shape can be used as a new probe of the turbulence. The 400 TeV and 2 PeV data sets of IceTop can be fitted with Goldreich-Sridhar turbulence and a broad resonance function, but other possibilities are not excluded. We then present our first numerical calculations of the CRA down to 3 TeV energies in 3D isotropic Kolmogorov turbulence. At these low energies, the large-scale CRA aligns well with the direction of local magnetic field lines around the observer. In this type of turbulence, the CR intensity is flat in a broad region perpendicular to field lines. Even though the CRA is quite gyrotropic, we show that the local configuration of the turbulence around the observer does result in the appearance of weak, "non-gyrotropic" small-scale anisotropies, which contain information on the local turbulence level.