Cosmic Ray Small Scale Anisotropies and Local Turbulent Magnetic Fields

TL;DR

This paper investigates how small-scale cosmic ray anisotropies at TeV energies can be explained by their scattering in turbulent magnetic fields, using numerical simulations of particle trajectories in magnetohydrodynamic turbulence.

Contribution

It introduces a numerical framework for analyzing cosmic ray propagation in turbulent magnetic fields and applies Liouville's theorem to connect turbulence effects with observed anisotropies.

Findings

Turbulent magnetic fields can produce small-scale anisotropies in cosmic ray arrival directions.

Numerical simulations show scattering in turbulence affects anisotropy patterns.

Theoretical framework links magnetic turbulence properties to observed cosmic ray anisotropies.

Abstract

Cosmic ray anisotropy has been observed in a wide energy range and at different angular scales by a variety of experiments over the past decade. However, no comprehensive or satisfactory explanation has been put forth to date. The arrival distribution of cosmic rays at Earth is the convolution of the distribution of their sources and of the effects of geometry and properties of the magnetic field through which particles propagate. It is generally believed that the anisotropy topology at the largest angular scale is adiabatically shaped by diffusion in the structured interstellar magnetic field. On the contrary, the medium- and small-scale angular structure could be an effect of non-diffusive propagation of cosmic rays in perturbed magnetic fields. In particular, a possible explanation of the observed small-scale anisotropy observed at TeV energy scale, may come from the effect of particle scattering in turbulent magnetized plasmas. We perform numerical integration of test particle trajectories in low-$\beta$ compressible magnetohydrodynamic turbulence to study how the cosmic rays arrival direction distribution is perturbed when they stream along the local turbulent magnetic field. We utilize Liouville's theorem for obtaining the anisotropy at Earth and provide the theoretical framework for the application of the theorem in the specific case of cosmic ray arrival distribution. In this work, we discuss the effects on the anisotropy arising from propagation in this inhomogeneous and turbulent interstellar magnetic field.