Unveiling Equatorial Ionospheric Response to May 2024 Mother's Day Geomagnetic Storm using Multi-Instrument Observations over India

Response of the equatorial and low-latitude ionosphere to the May 2024 super geomagnetic storm is investigated using coordinated ground and space-based measurements over the Indian sector. The observations revealed a strong negative ionospheric storm during both main and recovery phases, characterized by severe post-midnight reduction in NmF2 (or TEC) persisting for nearly ~6 hours. The density depletion during the main phase is attributed to the combined effect of westward penetration electric fields, which forced the density to lower altitudes enhancing recombination, together with thermospheric composition changes associated with equatorward meridional winds. Surprisingly, a more severe reduction in density was observed during the recovery phase, despite absence of strong IMF Bz fluctuations, suggesting additional influences of disturbance dynamo processes, TIDs and compositional changes. The severe density depletion in both nights occurred without significant change in h’F, making it a peculiar phenomenon. The nocturnal zonal plasma drifts exhibited significant deviations from quiet-time behaviour and showed pronounced anti-correlation with vertical drifts, likely associated with storm-time electrodynamics and possible presence of E region conductivity gradients. Further, abrupt dawn-time uplift in F-layer to ~850 km, accompanied by strong upward plasma drifts (~130 m/s), indicates the influence of eastward over-shielding penetration electric fields. During the recovery phase, unusual cusp-type F3 layers, significant iso-height and iso-frequency oscillations, and daytime positive storm effects were observed over the equator, associated with enhanced EEJ variations. Correlation and wavelet analysis further suggest that quasi-periodic DP2 electric field fluctuations significantly modulated the ionospheric density during the recovery phase.