Northern Crest Weakening Induces Large-Scale Hemispheric EIA Disappearance in GOLD Observations During a Storm

The equatorial ionization anomaly (EIA) is a crucial phenomenon for understanding the ionospheric dynamics. However, the discrepancies in observations from different instruments resulting from the north-south EIA differences remain unreported and understudied. Here, we present the first comprehensive observation of a large-scale single-peak EIA event, detected by the Global-scale Observations of the Limb and Disk (GOLD) mission from longitude ∼−80° to ∼−20° during a storm. However, total electron content (TEC) observations and Whole Atmosphere Community Climate Model-eXtended (WACCM-X) simulations show that the single-peak EIA observed by GOLD is not a real single-peak structure. WACCM-X simulations reveal that the rapid weakening of the northern crest west of 20°W makes the OI 135.6 nm emission undetectable by GOLD, resulting in an unrealistic single-peak EIA in GOLD observations. This weakening is attributed to downward plasma transport by neutral winds dominating over upward plasma transport by E $\times$ B, which lowers the F-region peak electron density height (hmF2) altitude and accelerates recombination. In addition, disturbance dynamo electric field caused by geomagnetic storms should have played an important role in the processes mentioned above. This study reveals unique EIA evolution dynamics and exposes limitations in single-data set observations, establishing multi-instrument validation as essential for EIA characteristic investigations.