Cosmic-Ray Convection-Diffusion Anisotropy

TL;DR

This paper investigates how nonuniform convection causes anisotropies in cosmic-ray distributions, linking these effects to fluid inertial and shear forces, and uses observational data to constrain local interstellar medium acceleration.

Contribution

It introduces a model connecting convection-related anisotropies in cosmic rays to inertial and shear forces, providing new insights into their energy dependence and observational constraints.

Findings

Anisotropies increase with cosmic-ray energy.

Upper limit on local interstellar medium acceleration is about 100 μm/s².

Quadrupole anisotropy at >200 TeV can be modeled with Galactic shear effects.

Abstract

Under nonuniform convection, the distribution of diffusive particles can exhibit dipole and quadrupole anisotropy induced by the fluid inertial and shear force, respectively. These convection-related anisotropies, unlike the Compton-Getting effect, typically increase with the cosmic-ray (CR) energy, and are thus candidate contributors for the CR anisotropy. In consideration of the inertial effect, CR observational data can be used to set an upper limit on the average acceleration of the local interstellar medium in the equatorial plane to be on the order of 100 $ \mu \text{m}/\text{s}^2 $. Using Oort constants, the quadrupole anisotropy above 200 TeV may be modeled with the shear effect arising from the Galactic differential rotation.