by K. Shimna, R. K. Archana, C. Vineeth, Gang Lu, T. K. Pant and M. S. M. Vijayan
During the recovery phase of the Gannon Superstorm on 11 May 2024, the thermosphere-ionosphere system experienced dramatic changes marked by an unusual redistribution of plasma density. The peak electron density (Nmax) from Global-scale Observations of the Limb and Disk (GOLD), ground-based Vertical Total Electron Content (VTEC), and topside VTEC from Swarm C satellite revealed an enhanced electron density at the equator and a complete disappearance of the Equatorial Ionization Anomaly (EIA) crests. This absence of EIA crests was linked to a strong daytime Counter Electrojet (CEJ) caused by disturbance dynamo effects observed in ground magnetic data. The unusually strong CEJ suppressed the ExB vertical drift, thereby eliminating the fountain effect. Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM) simulations further indicated the presence of equatorward converging meridional winds from both hemispheres. This converging wind hindered the diffusion of plasma from the dip equatorial rΣO/N2 ratio along the equator was observed during the recovery phase, further contributing to this phenomenon. The present study, therefore, for the first time, demonstrates the combined effects of storm-induced changes in equatorial electrodynamics, meridional winds, and significant compositional changes during the recovery phase of a superstorm, and their influence on plasma distributions. The corroborative evidence from model simulations further strengthened these observational findings.