Drift effects and the average features of cosmic ray density gradient in CIRs during successive two solar minimum periods

TL;DR

This study analyzes the spatial gradient of cosmic ray density in corotational interaction regions during two solar minimum periods, revealing drift effects and diffusion contributions through muon detector data.

Contribution

It introduces a method to deduce 3D cosmic ray density gradients from muon detector anisotropy and compares gradient features across different solar epochs.

Findings

Latitudinal gradient Gz changes sign across the HCS in accordance with drift predictions.

Negative enhancement in Gx observed after HCS crossing in both epochs.

Asymmetry in Gx and Gy suggests significant diffusion contributions alongside drift effects.

Abstract

We deduce on hourly basis the spatial gradient of the cosmic ray density in three dimensions from the directional anisotropy of high-energy (~50 GeV) galactic cosmic ray (GCR) intensity observed with a global network of muon detectors on the Earth's surface. By analyzing the average features of the gradient in the corotational interaction regions (CIRs) recorded in successive two solar activity minimum periods, we find that the observed latitudinal gradient (Gz) changes its sign from negative to positive on the Earth's heliospheric current sheet (HCS) crossing from the northern to the southern hemisphere in A<0 epoch, while it changes from positive to negative in A>0 epoch. This is in accordance with the drift prediction. We also find a negative enhancement in Gx after the HCS crossing in both A<0 and A>0 epochs, but not in Gy. This asymmetrical feature of Gx and Gy indicates significant contributions from the parallel and perpendicular diffusions to the the gradient in CIRs in addition to the contribution from the drift effect.