Cosmic ray short burst observed with the Global Muon Detector Network (GMDN) on June 22, 2015

TL;DR

This study analyzes a cosmic ray burst on June 22, 2015, using a global muon detector network to understand its anisotropy and density variations, revealing its association with the heliospheric current sheet crossing and ruling out geomagnetic storm effects.

Contribution

The paper provides the first detailed analysis of a cosmic ray burst using a global muon detector network, linking anisotropy changes to heliospheric structures and clarifying the cause of the burst.

Findings

CRB caused by local density maximum and anisotropy

Enhanced anisotropy normal to the HCS indicating diamagnetic drift

CR penetration into the inner heliosphere along the HCS

Abstract

We analyze the short cosmic ray intensity increase ("cosmic ray burst": CRB) on June 22, 2015 utilizing a global network of muon detectors and derive the global anisotropy of cosmic ray intensity and the density (i.e. the omnidirectional intensity) with 10-minute time resolution. We find that the CRB was caused by a local density maximum and an enhanced anisotropy of cosmic rays both of which appeared in association with Earth's crossing of the heliospheric current sheet (HCS). This enhanced anisotropy was normal to the HCS and consistent with a diamagnetic drift arising from the spatial gradient of cosmic ray density, which indicates that cosmic rays were drifting along the HCS from the north of Earth. We also find a significant anisotropy along the HCS, lasting a few hours after the HCS crossing, indicating that cosmic rays penetrated into the inner heliosphere along the HCS. Based on the latest geomagnetic field model, we quantitatively evaluate the reduction of the geomagnetic cut-off rigidity and the variation of the asymptotic viewing direction of cosmic rays due to a major geomagnetic storm which occurred during the CRB and conclude that the CRB is not caused by the geomagnetic storm, but by a rapid change in the cosmic ray anisotropy and density outside the magnetosphere.