Large-Scale Cosmic-Ray Anisotropy as a Probe of Interstellar Turbulence

TL;DR

This paper models large-scale cosmic-ray anisotropies using different interstellar turbulence theories and compares predictions with IceTop data, suggesting anisotropy observations can probe local interstellar turbulence properties.

Contribution

It provides the first detailed comparison of cosmic-ray anisotropy predictions from Goldreich-Sridhar and isotropic turbulence models with observational data.

Findings

GS model with smooth deficit of parallel waves explains cold spots

Isotropic fast magnetosonic turbulence matches high-energy anisotropy

Data can help probe interstellar turbulence properties near Earth

Abstract

We calculate the large-scale cosmic-ray (CR) anisotropies predicted for a range of Goldreich-Sridhar (GS) and isotropic models of interstellar turbulence, and compare them with IceTop data. In general, the predicted CR anisotropy is not a pure dipole; the cold spots reported at 400 TeV and 2 PeV are consistent with a GS model that contains a smooth deficit of parallel-propagating waves and a broad resonance function, though some other possibilities cannot, as yet, be ruled out. In particular, isotropic fast magnetosonic wave turbulence can match the observations at high energy, but cannot accommodate an energy dependence in the shape of the CR anisotropy. Our findings suggest that improved data on the large-scale CR anisotropy could provide a valuable probe of the properties - notably the power-spectrum - of the interstellar turbulence within a few tens of parsecs from Earth.