Extended Cosmic Ray Decreases with Strong Anisotropy after Passage of Interplanetary Shocks

Abstract

The passage of an interplanetary shock and/or interplanetary coronal mass ejection often causes a rapid decrease in the Galactic cosmic-ray (GCR) flux, known as a Forbush decrease, followed by a recovery of the flux over some days. These local effects are of short duration and strongly rigidity dependent, with higher-rigidity particles exhibiting much weaker effects. In contrast, we present data for two events in which the cosmic-ray flux gradually decreased for about 1 week after shock passage, then recovering over the following week, with the highest anisotropy levels observed throughout Solar Cycle 24. These extended decreases have a weak rigidity dependence and are much more prominent in observations at higher cutoff rigidity, where the initial Forbush decrease is not clearly detected and other variations are generally weak, as we demonstrate using data from the Princess Sirindhorn Neutron Monitor at Doi Inthanon, Thailand with a cutoff rigidity of about 17 GV. We propose that these extended decrease events were initiated upon the passage of an interplanetary shock that inhibited the inflow of GCRs along the interplanetary magnetic field, possibly due to magnetic mirroring at the shock. We also discuss the general behavior of GCR anisotropy as observed at this high cutoff rigidity.

Analysis of temporal variation of cosmic ray intensity observed with global networks of neutron monitors and muon detectors

“…In an analysis using the global network data, however, we need to pay a special attention to the variation due to the "local effect" arising from the environmental variation which is generally different in each detector. In this paper, we present our sample analyses of the "extended cosmic ray decreases" observed by NMs and MDs in January-February and July-August, 2012 [3]. We particularly show the local effect seen in MD data and discuss the analysis method to eliminate its influence on the derived GCR density and anisotropy variations.…”

“…More detail information of this analysis can be found in [6] in which we analyzed a significant space weather event during a few days in November 2021. In this study, we analyze two solar rotation periods in January-February and July-August, 2012, when the extended cosmic ray decreases in association with strong diurnal anisotropy were reported from the observation by the PSNM neutron monitor [3]. /24.…”