The sidereal anisotropy of multi-TeV cosmic rays in an expanding Local Interstellar Cloud

TL;DR

This paper investigates the anisotropy of multi-TeV cosmic rays in the local interstellar cloud, proposing a model involving magnetic field orientation and drift effects, and estimates the diffusion coefficient based on observations.

Contribution

It introduces a steady-state model of cosmic ray distribution in an expanding local interstellar cloud, linking anisotropy observations to diffusion and drift processes.

Findings

The anisotropy is explained by superimposing bi-directional and uni-directional flows.

The model estimates the perpendicular diffusion coefficient of multi-TeV GCRs.

The density gradient is consistent with an expanding Local Interstellar Cloud.

Abstract

The sidereal anisotropy of galactic cosmic ray (GCR) intensity observed with the Tibet Air Shower (AS) experiment still awaits theoretical interpretation. The observed global feature of the anisotropy is well reproduced by a superposition of the bi-directional and uni-directional flows (BDF and UDF, respectively) of GCRs. If the orientation of the deduced BDF represents the orientation of the local interstellar magnetic field (LISMF), as indicated by best-fitting a model to the data, the UDF deviating from the BDF orientation implies a significant contribution from the streaming perpendicular to the LISMF. This perpendicular streaming is probably due to the drift anisotropy, because the contribution from the perpendicular diffusion is expected to be much smaller than the drift effect. The large amplitude deduced for the UDF indicates a large spatial gradient of the GCR density. We suggest that such a density gradient can be expected at the heliosphere sitting close to the boundary of the Local Interstellar Cloud (LIC), if the LIC is expanding. The spatial distribution of GCR density in the LIC reaches a stationary state because of the balance between the inward cross-field diffusion and the adiabatic cooling due to the expansion. We derive the steady-state distribution of GCR density in the LIC based on radial transport of GCRs in a spherical LIC expanding at a constant rate. By comparing the expected gradient with the observation by Tibet experiment, we estimate the perpendicular diffusion coefficient of multi-TeV GCRs in the local interstellar space.