Observations and Modelling investigations of Ionospheric Response to April 23-24, 2023, G4-Class Geomagnetic Storm over Indian Sector

This study explores the ionospheric response over the Indian sector to the G4-class geomagnetic storm of April 23-24, 2023. Utilizing multi-instrument observations and SAMI2 modeling, ionospheric behavior was examined during the storm's main phase (17:41 UT, April 23 - 04:03 UT, April 24) and the recovery phase (04:03 UT - 22:44 UT, April 24). During the main phase, ionosonde data from Tirunelveli showed rapid F-layer height (h’F) variations driven by westward and eastward prompt penetration electric fields (PPEFs). The westward PPEF, induced by undershielding, led to an initial decrease in h’F followed by an increase, suppressing pre-existing Equatorial Plasma Bubbles (EPBs) within two hours of the storm’s onset. Despite a late-night rise in h’F due to overshielding, no new EPB formed. The recovery phase exhibited a positive storm effect at low latitudes and a negative effect at higher latitudes, linked to disturbance dynamo electric fields (DDEFs) and thermospheric composition changes (O/N₂). Isofrequency analysis of CADI ionosonde and GNSS TEC data revealed large-scale traveling ionospheric disturbances (LSTIDs) with a ~2-hour period, ~2,450 km wavelengths, and ~340 m/s equatorward propagation speed. These LSTIDs were likely driven by atmospheric gravity waves or auroral heating. The westward DDEF suppressed the equatorial ionization anomaly (EIA) and inhibited post-sunset EPBs, while eastward DDEF increased h’F post-midnight without EPB formation. We speculate this absence might be due to a lack of seeding mechanisms. SAMI2 simulations incorporating E×B drift data reproduced several storm-time features in the main and recovery phases.