Characterizing Severe Geomagnetic Storms and Their Magnetospheric Drivers in Solar Cycle 23 & 24

This study presents a systematic statistical analysis of the interplanetary drivers of severe geomagnetic storms (Dst ≤ -150 nT) over Solar Cycles 23 and 24. Utilizing OMNI plasma and magnetic field data, we investigate the relationship between storm intensity, key solar wind parameters (IMF B, Bz, Vsw, Nsw, Tsw), and solar activity proxies (SSN, F10.7). Our analysis confirms that all severe storms were unequivocally associated with prolonged intervals of southward IMF-Bz, affirming magnetic reconnection as the primary trigger mechanism. We find that the majority of events (~ 75%) are driven by four distinct structures: magnetic clouds (MC) and sheath fields (Sh) are the most prevalent, each responsible for 30.4% of storms, followed by combined sheath-MC structures (Sh + MC, 17.4%) and non-cloud ICMEs (17.4%). The occurrence and dominance of these drivers exhibit a strong solar cycle dependence: sheath fields and MCs prevail during the rising phase, sheath dominance peaks at solar maximum, while MCs, sheaths, and corotating interaction regions (CIRs) are the main drivers in the declining phase. Correlation analysis reveals a dominant control exerted by the peak parameters of the Interplanetary Magnetic Field (IMF), specifically the total magnetic field magnitude (B peak, CC = 0.65) and its crucial north-south component (Bz peak, CC = 0.60). These strong positive correlations highlight the direct and immediate influence of the IMF structure on storm severity. Moderate and weak relationships were observed with other solar wind parameters, including temperature (CC = -0.50) and density (CC = -0.19). Crucially, despite their fundamental role in the space environment, proxies for overall solar activity, such as the F10.7 radio flux (CC = -0.36) and sunspot number R (CC = -0.23), exhibit significantly weaker correlations with Dst peak. In stark contrast, however, geomagnetic activity indices like Kp (CC = -0.70) and Ap (CC = -0.74) show a strong negative correlation, reflecting their nature as results of the same storm phenomenon rather than its direct cause. Collectively, these findings underscore that while solar activity modulates the frequency of space weather events, the inherent physical properties of the interplanetary structures, especially the intensity and duration of southward Bz, are the most direct and potent determinants of severe geomagnetic storm intensity.