Indian Equatorial Ionosphere Under the Influence of the May 2024 Geomagnetic Storm: Insights Into Electric Fields, Plasma Irregularities and F-Region Dynamics

This study investigates the distinctive features of the daytime equatorial E-region ionosphere using observations of HF radar (18 MHz), ionosonde, and magnetometer at Thumba (8.5°N, 77°E, dip lat = 1.9°N), India, during the extreme geomagnetic storm of May 10–11, 2024 (minimum SYM-H ∼ −497 nT). The northward component of the Interplanetary Magnetic Field (IMF Bz) exhibited large-amplitude oscillations throughout the day on May 11 that resulted in sudden changes in the Interplanetary Electric Field, in turn manifesting as Prompt Penetration Electric Field (PPEF) in the ionosphere. The drift measurements of plasma irregularities using the radar indicate that these PPEF modulations effectively mapped onto the equatorial ionosphere in the Indian region. The radar observations of the zonal drift of daytime E-region plasma irregularities exhibited four notable and significant aspects: (a) enhancements in westward drift velocities during eastward electric field to 500 m/s, exceeding the ion acoustic limit by ∼140 m/s, (b) eastward reversal of drift with speeds ∼200 m/s in response to westward PPEF, (c) occasional disappearance of plasma irregularities, and (d) short-lived eastward drifting echoes in east beam of radar suggesting a possible localized alteration in electron density profiles. In the absence of PPE15F modulation, one of the significant enhancements in the drift of plasma irregularities coincided with an X5-class solar flare. Concurrent ionosonde measurements revealed a rise in the ionospheric peak altitude beyond 500 km during local noon, accompanied by the presence of the F₃ layer. This alluded to the combined effect of eastward PPEF and equatorward thermospheric wind.