Low-Latitude Ionospheric Responses to Two Matched Intense Equinoctial Geomagnetic Storms: A Case-Based Comparison Between Solar Cycles 24 and 25

Abstract

This study investigates the ionospheric responses to two matched intense equinoctial geomagnetic storms that occurred during Solar Cycles 24 and 25 (SCs 24 and 25), with emphasis on variations in Total Electron Content (TEC), the F2‑layer critical frequency (foF2), and the F2‑layer peak height (hmF2). TEC data were derived from Global Ionospheric Maps (GIMs) based on Global Navigation Satellite System (GNSS) observations, while foF2 and hmF2 parameters were obtained from the Global Assimilative Model of the Bottomside Ionosphere Timeline (GAMBIT) for 22 low‑latitude locations worldwide. The results show that the SC25 storm produced stronger and more spatially extensive ionospheric responses than the SC24 event. Peak TEC values during SC25 exceeded those of SC24 by up to ~80‑90 TECU in the Southeast Asian, East Asian, and Pacific sectors near the storm main phase, indicating a pronounced positive ionospheric storm. These enhancements are closely associated with higher solar wind speeds (~650‑700 km s–1) and strongly fluctuating IMF Bz during SC25, which generated sustained multi‑pulse Prompt Penetration Electric Fields (PPEFs) that intensified E × B plasma drifts and the equatorial fountain effect. In contrast, SC24 was characterized by lower solar wind speeds (~550‑600 km s–1) and a predominantly southward IMF Bz, resulting in shorter‑lived PPEF activity, weaker TEC enhancements, and pronounced depletion during the recovery phase. Correlation analysis between Dst and TEC disturbances reveals strong negative correlations (r ≈ −0.6 to −0.9) in the Asia‑Pacific sectors during the main phase, indicating that enhanced TEC disturbances tend to coincide with periods of increasing storm intensity, whereas predominantly positive correlations (r ≈ 0.6‑0.9) are observed in the Middle Eastern‑African‑European sector, reflecting TEC depletion. The foF2 response shows clear phase dependence, with localized daytime enhancements during the main phase but widespread reductions of ~15‑20% during recovery in both solar cycles, consistent with the influence of Disturbance Dynamo Electric Fields (DDEFs), and thermospheric composition changes. In contrast, hmF2 exhibits solar‑cycle‑dependent behavior, with modest increases (~3‑8%) during SC24 and more variable, often negative responses (up to ~2‑3% decrease) during SC25. These findings highlight the heterogeneous, region‑dependent, and solar cycle‑dependent nature  of ionospheric variability during intense geomagnetic storms, with important implications for space weather modeling and prediction.