Geomagnetic Cutoff Rigidity Variations Impact Cosmic Ray Flux During Forbush Decreases

This study investigates the effects of geomagnetic cutoff rigidity variations during Forbush decreases (FDs), which are short-term reductions in galactic cosmic ray flux caused by interplanetary disturbances. The research highlights that during some interplanetary coronal mass ejection (ICME) events, neutron monitor (NM) data can be affected by geomagnetic storms, leading to variations in the measured cosmic ray flux. These localized anomalies can extend to lower rigidities and reach 1 GV in some events, potentially biasing the rigidity dependence inferred from NM-based hourly proton spectra. To address this issue, the study utilizes data from the Alpha Magnetic Spectrometer (AMS), which measures proton rigidity directly in space and is not affected by cutoff variations at ground stations. By using AMS data as a stable reference, the researchers are able to constrain corrections for selected events, removing localized anomalies while preserving the broader FD evolution. The results demonstrate that short-timescale cosmic ray variability during FDs reflects both heliospheric modulation and storm-time changes in geomagnetic shielding. This research underscores the importance of accounting for geomagnetic effects when analyzing cosmic ray data, particularly during disturbed intervals. By incorporating AMS data and applying appropriate corrections, scientists can obtain more accurate measurements of cosmic ray flux and improve our understanding of space weather phenomena.

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