Forbush decreases during strong geomagnetic storms: time delays, rigidity effects, and ICME-driven modulation

We present the relationship between Forbush decreases (FDs) and associated geomagnetic storms, as well as their connections to interplanetary (IP) solar wind parameters, using high resolution minute data. FDs were classified into groups based on main phase decrease steps, and each group was analyzed using superposed epoch analysis. The results reveal that fast, turbulent, high-field sheath structures form before and pass during the onset of coronal mass ejection (CME) driven FDs, whereas corotating interaction region (CIR) driven events exhibit delayed amplification and more perturbed dynamics. Time lags between the onset of FDs and geomagnetic storms were calculated and discussed, providing insights crucial for space weather forecasting. Correlation analyses between FD amplitude and peak values of various IP parameters were performed and discussed. The relationship between FDs and geomagnetic storms was analyzed, revealing that for CME-driven events, FD amplitudes exhibit a stronger correlation with moderate and strong geomagnetic storms compared to extreme storms. The weaker correlation during extreme CME-driven storms may result from complex magnetospheric responses caused by successive events and prolonged southward interplanetary magnetic field Bz, unlike the more direct responses observed in moderate and strong single-event storms. Interplanetary coronal mass ejection (ICME) manifestations were also correlated with FD amplitude, showing that events with fast forward shocks and compression sheath regions exhibit stronger correlations than those without shocks. Furthermore, we analyzed the energy dependence of FD amplitude using data from twelve neutron monitor stations at different latitudes and altitudes across the globe. As a result, the cosmic ray (CR) energy spectrum exhibits a two-step linear dependence with the FD amplitude, in the lower rigidity FD amplitude decreases sharply, while in higher rigidity regimes, the decrease is more gradual. A broader energy spectrum is recommended for more comprehensive conclusions.