Reversed Interhemispheric Asymmetry of the Equatorial Ionization Anomaly during the September 2020 Equinox in the American Sector: Observational and Simulation Evidence of Lower-Atmospheric Forcing

The Equatorial Ionization Anomaly (EIA) is a notable characteristic of the ionospheric F-region near the geomagnetic equator, characterized by two electron density peaks around ±15° magnetic latitude and a trough at the equator. During equinox periods in the ~75°W longitude sector, the EIA peak in the Northern Hemisphere is stronger on average, a pattern attributed to combined effects of prevailing neutral winds and geomagnetic field configuration. However, occasional reversals occur where the peak in the Southern Hemisphere is stronger, which may be associated with the lower atmospheric forcing. The specific lower-atmospheric forcings driving this reversed interhemispheric asymmetry (IHA) of the EIA, and their relative contributions, remain insufficiently understood. Using GNSS-derived TEC data over the 75°W longitudinal sector, we identified a two-day geomagnetically quiet period around the September equinox in 2020, during which the IHA of the EIA reverses, underscoring the potential role of lower atmospheric forcing in modulating the IHA on a day-to-day basis. The specified dynamics version of the Whole Atmosphere Community Climate Model with thermosphere-ionosphere extension (SD-WACCMX) has been used to explore the formation of a strong SH EIA peak. The simulation successfully reproduced the key observed features, and the respective contributions of major lower thermospheric forcings are analyzed. This study provides new insights into the IHA of the EIA and its linkage to the lower atmospheric forcings.