Convection Anisotropies of Cosmic Rays in Highly Magnetized Plasma

TL;DR

This paper extends a turbulent convection model for cosmic ray anisotropies to include anisotropic diffusion, explaining observed small-scale anisotropies in TeV cosmic rays through magnetic turbulence effects.

Contribution

It introduces an anisotropic diffusion extension to the convection model, accounting for magnetic field effects on cosmic ray anisotropies, and explains the observed power-law spectrum.

Findings

The angular power spectrum follows a power-law with index related to turbulence spectral index.

Anisotropic diffusion explains small-scale anisotropies in TeV cosmic rays.

The model aligns with Kolmogorov turbulence law for magnetic fluctuations.

Abstract

Recently, Zhang & Liu (2024) proposed a turbulent convection model for multiscale anisotropies of cosmic rays (CRs), with an assumption of isotropic diffusion such that the anisotropies are statistically isotropic. However, this assumption may be unrealistic for TeV CRs, whose observations have revealed the significance of the local interstellar background magnetic field. To meet the difficulty, the turbulent convection scenario needs to be extended to cover anisotropic diffusion. In this paper, we focus on the parallel diffusion with isotropic pitch-angle scattering, which may be an approximation to the transport process driven by weak hydromagnetic waves in a magnetic flux tube, where fluctuations of the wave velocities lead to the turbulent convection. The consequence is the breaking of the statistical isotropy, while the overall shape of the angular power spectrum, $ \overline{C_\ell}\propto\ell ^{-\gamma -1} $ ($ \ell\gg 1 $), remains similar to that in the isotropic diffusion model, where $ \ell $ are degrees of spherical harmonics, and $ \gamma $ is the turbulence spectral index of the convection field. It is then expected that the power-law index of the TeV CR small-scale angular power spectrum can be explained with the Kolmogorov law $ \gamma =5/3 $, irrespective of the background magnetic field to some extent.