Cosmic-Ray Short Burst Observed with the Global Muon Detector Network (GMDN) on 2015 June 22

Abstract

We analyze the short cosmic-ray intensity increase ("cosmic-ray burst": CRB) on 2015 June 22 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 cutoff rigidity and the variation of the asymptotic viewing direction of cosmic rays due to a major geomagnetic storm that 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.

A Peculiar ICME Event in August 2018 Observed With the Global Muon Detector Network

“…A similar idea was also presented by Mohanty et al (2016) to interpret the "cosmic-ray short burst" observed by a muon detector in June 2015. Based on calculations of cosmic-ray trajectory in the latest model of geomagnetic field, however, analyses of the same event by Munakata et al (2018) showed that the reductions of P c and the magnetic deflection of cosmic-ray orbits are not enough to cause the observed intensity increase of 60 GeV cosmic rays monitored by the GMDN, because the GMDN only has a small response to cosmic rays with rigidities around P c . They attributed the burst to enhancements of I 0 (t) and ξ(t) outside the magnetosphere caused by Earth's crossing the heliospheric current sheet (Munakata et al, 2018).…”

Peculiar Solar Sources and Geospace Disturbances on 20–26 August 2018

“…In particular, these effects can lead to a short enhancement or burst of the CR count rate on the background of the main FD observed at some neutron monitors (NMs). Several such events recorded during Cycle 24 are analyzed in the recent papers by Evenson et al (2017), Gil et al (2018), Munakata et al (2018), Samara et al (2018). Even those CR bursts that stand out well at the FD recovery phase do not usually reach the pre-event count rate level.…”

A Peculiar ICME Event in August 2018 Observed with the Global Muon Detector Network

“…A small local maximum is seen in I 0 (t) at around 10:00 UT of 25 August being superposed on the gradual decrease in the sheath period. We think that this is probably due to the discontinuity recorded in the IMF magnitude and longitude seen in Figures 1c-d, because such a local increase of I 0 (t) is often observed by the GMDN at the IMF discontinuity (Munakata et al, 2018).…”