Relationship between Solar Wind Speed at Shock Arrival and Interplanetary Coronal Mass Ejections Start Time during Intense Geomagnetic Storms

Abstract

This study provides a comparative investigation of solar wind speed at shock arrival (V₁), solar wind speed at the interplanetary coronal mass ejection start time (V₂), and the minimum southward interplanetary magnetic field (IMF Bz) in relation to intense geomagnetic storms during solar cycle (SC) 24 and the early phase of SC 25 (2020–2023). Intense geomagnetic storms were selected using the criterion disturbance storm time (Dst) ≤ –100 nT. For each event, V₁, V₂, IMF Bz, and interplanetary coronal mass ejection (ICME) duration were analyzed to evaluate their relative contributions to geomagnetic storm intensity. Comparative statistical analyses were further performed across different phases of SC 24 and seasonal periods. The results show that geomagnetic storm intensity is controlled by the combined effects of solar wind speed, the magnitude and persistence of southward IMF Bz, and the duration of ICME–magnetosphere interaction. Several events characterized by moderate solar wind speeds produced intense geomagnetic storms when accompanied by prolonged southward IMF Bz and extended ICME durations, indicating that sustained solar wind–magnetosphere coupling is a key factor in storm development. Comparisons among events during the ascending, maximum, and descending phases of SC 24 revealed that longer ICME durations can produce stronger geomagnetic storms even under relatively weaker solar wind conditions. The analysis of early phase of SC 25 events further demonstrated that similar geomagnetic storm intensities may arise from different combinations of controlling parameters, highlighting compensating effects between solar wind speed, IMF Bz, and interaction duration. The statistical analysis showed that CME and halo CME (HCME) occurrence rates peaked during the solar maximum phase, whereas some of the most intense geomagnetic storms occurred during the descending phase of SC 24. This indicates that CME frequency alone is insufficient to explain geomagnetic storm intensity and that intrinsic CME properties and solar wind coupling efficiency play important roles in determining geoeffectiveness. Seasonal analysis revealed that intensity of geomagnetic activity peaks in autumn and decreases in summer, suggesting that storm occurrence varies with seasonal factors and the phase of the SC. The relationship between sunspot number and geomagnetic storm occurrence was found to be complex and non-linear, suggesting that sunspot number alone is not always a reliable indicator of geomagnetic storm activity. Overall, the results confirm that intense geomagnetic storms are governed by multiple interacting solar wind and interplanetary parameters rather than by a single controlling factor. The findings improve the understanding of CME geoeffectiveness and solar wind–magnetosphere coupling processes and provide useful insight for improving space weather forecasting and prediction of extreme geomagnetic storm events.