Effects of Geomagnetic Cutoff Rigidity Variations during Forbush Decreases

Abstract: Forbush decreases (FDs) are short-term reductions in galactic cosmic ray flux caused by interplanetary disturbances. During some interplanetary coronal mass ejection (ICME) events, neutron monitor (NM) data also contain variations produced by geomagnetic storms. Earlier studies emphasized apparent effects near 10~GV, but storm-time changes in geomagnetic cutoff rigidity can either increase or decrease the ground-level count rate. Using a recently published hourly proton flux reconstructed from NM data for May 2011 through October 2019, the interval covered by the published AMS daily proton fluxes, we show that these localized anomalies can extend to lower rigidities and reach 1~GV in some events. Such effects can bias the rigidity dependence inferred from NM-based hourly proton spectra during disturbed intervals. Because AMS measures proton rigidity directly in space, its daily proton spectrum is not affected by cutoff variations at ground stations and provides a stable reference. We therefore use AMS to constrain corrections for selected events. The correction removes localized anomalies while preserving the broader FD evolution, and for a representative ICME event it brings the corrected daily averages closer to the AMS measurements. Our results show that short-timescale cosmic ray variability during FDs reflects both heliospheric modulation and storm-time changes in geomagnetic shielding.