Some Interesting Observations of Cosmic Ray Variations during the Extreme Solar Storm on 2024 May 10–11

Multiple interplanetary coronal mass ejections (ICMEs) erupted from AR 13664 during 2024 May 8–13. On 2024 May 10, after the arrival of the ICME shock at 17:04 UT, a Forbush Decrease (FD) was seen in various Neutron Monitor (NM) stations located worldwide. This paper presents a comprehensive analysis of neutron data from 28 NM stations and gamma-ray data obtained from the NaI(Tl) detector at Maitri, Antarctica. The present paper reports for the first time about an ICME shock-sheath related enhancement of ∼3%–5% magnitude in the neutron flux at stations with geomagnetic rigidity between 4 and 7 GV, located at higher altitudes (≥1000 m). For these stations, the high-rigidity ends of the asymptotic trajectories are confined to the midnight to the postmidnight region. It appears that the increase may be associated with magnetospheric particle precipitation triggered by the passage of the ICME shock-sheath. The variation of FD amplitude with rigidity shows a decreasing trend in general, with a minimum amplitude at 5–7 GV, which we attribute to the location of field-aligned currents (FACs). The FD amplitude is found to have the effects of storm-time westward ring current, FACs, and magnetospheric dynamics, in addition to the ICME parameters. During the recovery phase of FD, enhancement is observed between 01:50 and 03:00 UT on 2024 May 11, which coincided with Ground Level Enhancement (GLE#74). The GLE # 74 was coeval with the strong SYM-H index and the sudden recovery of the FD due to reduced interplanetary magnetic field, which might be responsible for the observed enhancement at high rigidity stations.