Quantitative Assessment of Geographic Control of December Solstice EIA Crest Shifts Over Southeast Asia Under Low Solar Activity

Abstract

This study quantitatively assesses the drivers of the Equatorial Ionization Anomaly (EIA) morphology and latitudinal shifts over Southeast Asia during the December solstice. We use a 27‐year ionospheric data set (1998–2024) of total electron content (TEC) and F2‐layer peak electron density (NmF2) under geomagnetically quiet conditions. Crest parameters are extracted using a dual‐Gaussian fitting method, and variance contributions are decomposed from solar and geographic factors through multiple linear regression. Geographic control, arising from asymmetric solar illumination relative to the geomagnetic equator, shows a statistically significant association with EIA morphology during periods of low solar activity (F10.7 < 100). The near 10° offset between the geographic and geomagnetic equators corresponds to southward subsolar migration, which coincides with illumination asymmetry and correlates with coherent southward EIA shifts. Southern crests displace about 9.0° southward (TEC) and 7.1° southward (NmF2) from the June to December solstice, nearly 1.8–3 times the displacement of the northern crests. Under low solar activity (F10.7 < 100), the subsolar latitude (geographic control) accounts for 98%–99% of the explained variance in southern crest displacement, whereas solar EUV flux (F10.7) contributes ≤2%. Adjusted R ² values (0.33–0.48) further indicate that 52%–67% of the total variance remains unexplained, a discrepancy likely attributable to E  ×  B drifts, neutral winds, thermospheric influences, and electrodynamic coupling processes. These findings highlight the statistically significant role of geographic control in shaping EIA asymmetry under weak solar forcing and provide quantitative constraints for improving regional ionospheric models and space weather forecasts in Southeast Asia.