Real-time monitoring of the rigidity spectrum of large Forbush decreases in May and October 2024 with the paired neutron monitor and muon detector at the Antarctic Syowa Station

Abstract

Large Forbush decreases (FDs) in the Galactic cosmic ray flux were observed by the paired neutron monitor (SNM) and the vertical channel of the muon detector (SMDV) at the Antarctic Syowa Station in May and October, 2024. The maximum count rate depressions in SNM and SMDV are ∼10% and ∼5% in the May FD, respectively. In the October FD, the observed count rates decreased in two steps following two successive interplanetary shocks. The maximum count rate depression recorded by SNM (SMDV) is ∼7% (∼3%) in the first step, while it is ∼8% (∼5%) in the second step. It is demonstrated that the SNM count rate fraction relative to the total count rate in SNM and SMDV (Fn) is a good real-time indicator of the temporal variation of the cosmic ray rigidity spectrum (ΔΓ(P,t)), which is an important parameter for identifying the physical processes responsible for FD effects. It is verified that the variation of Fn is consistent with ΔΓ(P,t) calculated from the Global Fitting Analysis (GFA) at the Syowa Station. Fn also shows a significant increase of ΔΓ(P,t) during the ground-level enhancement (GLE) due to solar energetic particles on May 11 superposed on the FD recovery phase, implying that the total (Galactic + solar) cosmic ray spectrum is softened due to the contribution from GLE intensity steeply increasing with decreasing rigidity. This implies that Fn can be also a good indicator of small GLEs which are sometimes difficult to identify in the count rate variation when masked by an FD and the associated diurnal anisotropy. Graphic Abstract: (Figure presented.)