Testing the Rigidity Dependence of Galactic Cosmic Ray Intensity Variation using the Algorithm-Selected Forbush Decreases

The Earth is continuously receiving streams of charged particles originating from various sources in the cosmos. Through their interactions with the Earth‘s magnetic field, galactic cosmic rays (GCRs)have significant effects on terrestrial systems and space weather. We investigate the rigidity dependence of GCR intensity variations using algorithm-selected For-bush decreases (FDs) data from daily averaged GCR count stored at Thule (THUL), Mcmurdo (MCMD), Hermanus (HRMS), Calgary (CARG), Oulu (OULU), Apatity (APTY), Climax (CLMX), Haleakala (HLEA), and Athens (ATHN) neutron monitor (NM) stations between 1996 and 2008. A breakdown of the catalogues shows that 3126 (∼ 68%) are small [F D(%) > −3]while 1487 (∼ 32%) are large [F D(%) ≤ −3] FDs. Only 22 FDs (∼ 0.5%), identified using a R-FD coincident algorithm, were found to be simultaneously detected at the nine NM stations. A further analysis of the simultaneous FDs shows that the mean FD amplitudes in individual stations was inversely correlated (with r =-0.91) with the cut-off rigidities. NMs with low cut-off rigidity demonstrated high FD amplitudes while stations with the highest cut-off rigidity demonstrated low FD amplitudes. Our observation indicate that higher geographical latitudes are associated with higher FDamplitudes in NMs. The regression analyses of the relationships between FD amplitudes in individual NMstations and geomagnetic and interplanetary parameters was tested. The analyses show that inter planetarymagnetic field (IMF), solar wind speed (SWS), disturbance storm time index (Dst) and planetary A index(Ap) are the main drivers of CR flux intensity variation at the time of FD. At 95% confidence level, the regression analyses showed statistically significant results.