Equatorial ionospheric plasma bubbles during intense geomagnetic storms of Solar Cycle 25

Abstract

This study examines the low-latitude ionospheric response to four intense geomagnetic storms during Solar Cycle 25 (March, April, November 2023, and May 2024), focusing on Equatorial Ionization Anomaly (EIA) variations and post-sunset plasma irregularities. We used the model for Joule Heating (JH), Madrigal total electron content (TEC) maps, and GNSS-derived ROTI to analyze storm-time changes in EIA structure and equatorial plasma bubbles (EPBs). The May 2024 storm exhibited the strongest post-sunset JH, particularly near the June solstice, while March and April storms showed moderate JH and November the lowest. Equinox storms produced nearly symmetric JH patterns, while solstice storms revealed interhemispheric asymmetries. Following JH thresholds are used for the classification of storms: weak (20–30 mWm-2, November), moderate (30–50 mWm-2, March/April) and strong (>50 mWm-2, May).JH, together with storm-time electric fields and equatorial meridional winds, influence the location, strength, hemispheric asymmetry, and the generation or suppression of plasma irregularities of the EIA crest. The generation of ionospheric plasma irregularities and their geographical distribution strongly depend on EIA's density gradients and general structure. Well-developed double-crest EIAs with steep density gradients favor post-sunset irregularities, while single-crest or merged EIAs are less favorable. Fluctuations in the IMF Bz drive east-west prompt penetration electric fields that dynamically modulate the F region, altering the plasma fountain effect, the EIA structure, and the distribution of plasma bubbles after sunset. These results suggest that during geomagnetic storms, the combined effects of storm-driven electrodynamics and neutral winds modulate low-latitude ionospheric variability, influencing EIA dynamics and the formation of plasma irregularities.