Anomalous Negative H-Component Perturbations during Sudden Storm Commencements: Evidence and Space Weather Implications across Solar Cycles 23–25 [Dataset]

Sudden Storm Commencements (SSC), indicating the onset of geomagnetic storms, are typically characterized by a global positive spike in the H-component. However, this study reveals a previously unrecognized longitudinal anomaly during SSC associated with intense substorms (AE < −2500 nT). Distinct negative H-component perturbations are observed in regions west of the line (longitude) connecting the subsolar point and geomagnetic poles on the sunlit side, spanning geomagnetic latitudes of 20°-60°. Additionally, geomagnetically induced currents consistently peak in the mid and sub-auroral (45°-65°) American sector, highlighting its elevated space weather vulnerability in those regions. A set of conjectures is proposed, suggesting that complex Solar Wind-Magnetosphere-Ionosphere coupling processes, possibly involving asymmetric current systems and waves, may drive these anomalies. These results provide new insights into morphology of SSC and raise questions about the conventional understanding of the sequence of electrodynamic processes triggered by interplanetary shocks and magnetic reconnection during extreme space weather events.