Two Numerical Methods for the 3D Anisotropic Propagation of Galactic Cosmic Rays

TL;DR

This paper introduces two numerical methods to solve the 3D anisotropic cosmic ray propagation equation, addressing limitations of isotropic models and improving predictions of cosmic ray anisotropy and spectra.

Contribution

It presents two novel numerical schemes, the pseudo source method and Hundsdorfer-Verwer scheme, for modeling anisotropic cosmic ray transport in three dimensions.

Findings

Both methods successfully reproduce observed B/C and proton spectra.

The schemes accurately model the radial variation of spectral index.

Preliminary dipole anisotropy predictions align with observations.

Abstract

Conventional cosmic-ray propagation models usually assume an isotropic diffusion coefficient to account for the random deflection of cosmic rays by the turbulent interstellar magnetic field. Such a picture is very successful in explaining many observational phenomena related to the propagation of Galactic cosmic rays, such as broken power-law energy spectra, secondary-to-primary ratios, etc. However, the isotropic diffusion presupposition is facing severe challenges from recent observations. In particular, such observations on the large-scale anisotropy of TeV cosmic rays show that the dipole direction differs from the prediction of the conventional model. One possible reason is that the large-scale regular magnetic field, which leads to an anisotropic diffusion of cosmic rays, has not been included in the model provided by the public numerical packages. In this work, we propose two numerical schemes to solve the $3$-dimensional anisotropic transport equation: the pseudo source method and Hundsdorfer-Verwer scheme. Both methods are verified by reproducing the measured B/C and proton spectrum and the radial variation of spectral index expected by former 2D simulation. As a demonstration of the prediction capability, dipole anisotropy is also calculated by a toy simulation with a rough magnetic field.