Forbush Decreases during strong Geomagnetic Storms: Time Delays, Rigidity Effects, and ICME-Driven Modulation

Abstract: We investigate the relationship between Forbush decreases (FDs) and associated geomagnetic storms, and their links to interplanetary solar wind parameters, using high-resolution minute data. FDs are classified by main-phase decrease steps and analyzed with superposed epoch analysis. Fast, turbulent, high-field sheath structures occur before and during coronal mass ejection (CME)-driven FDs, whereas corotating interaction region events show delayed amplification and more perturbed dynamics. Time lags between FD and storm onsets are examined for space weather forecasting. FD amplitude correlates more strongly with moderate and strong CME-driven storms than with extreme storms, likely due to complex magnetospheric responses from successive events and prolonged southward IMF Bz. Events with fast shocks and sheath regions show stronger correlations than those without shocks. Energy dependence, derived from twelve neutron monitor stations worldwide, reveals a two-step linear rigidity spectrum: sharp FD amplitude decrease at low rigidity and a more gradual drop at higher rigidity.