Global IsUG Index Maps for Tracking Ionospheric Variability: A Case Study of the 4-5 November 2023 Geomagnetic Storm

In this study, we analyze ionospheric and thermospheric changes during a composite geomagnetic storm on 4-5 November 2023 with two activity periods. On 4 November, the corotating interaction region (CIR) compression resulted in moderate activity (SYM-H = -60 nT), while on 5 November the arrival of two Coronal Mass Ejections (CMEs) embedded into the ongoing CIR caused intense geomagnetic disturbances (SYM-H = -188 nT). Using global observations of vertical total electron content (VTEC) from the Universitat Politècnica de Catalunya (UPC) Global Ionosphere Maps (GIMs) and the normalized Ionospheric storm scale from UPC GIMs (IsUG) storm-time index, we examine ionospheric anomalies during these events. The IsUG index reveals that both events began with positive VTEC changes, but their delayed responses differed significantly. During the 4 November event, the negative phase did not develop, while the 5 November storm produced severe negative VTEC anomalies. Using complementary satellite observations of thermospheric composition, temperature and density, along with ionosonde-derived large-scale traveling ionospheric disturbances (LSTIDs) velocities, we show that these contrasting responses were caused by differences in thermospheric heating and composition. On 4 November, high-latitude heating was weak, while in the 5 November storm it was significant and led to the development of global disturbance winds and strong O/N2 decreases, causing severe negative VTEC anomalies. Despite the complicated morphology, the IsUG index could track these dynamics in agreement with the underlying physical processes, and can be used for monitoring of complex ionospheric storms.