Post-Sunset Merging of Equatorial Ionization Anomaly Crests Into a Single Peak Over ∼35° Longitude During the 19 January 2026 Geomagnetic Superstorm

We investigate the nighttime equatorial ionosphere during the 19 January 2026 geomagnetic storm using Global-scale Observations of the Limb and Disk OI 135.6 nm observations and ground-based GPS Total Electron Content. The observations reveal a pronounced longitudinal asymmetry in the post-sunset equatorial ionization anomaly (EIA). Over the South American sector, the two EIA crests converged and merged into a single equatorial peak across similar to 35 degrees longitude within similar to 2 hr, while remaining separated with active equatorial plasma bubbles (EPBs) over the Atlantic and West African sectors. This behavior suggests a rapid, large-scale post-sunset crest merging under storm-time conditions. Concurrent radar and magnetometer measurements indicate storm-time westward electric fields drove the large-scale merging. The timing of these electric fields relative to local sunset suppressed prereversal enhancement and inhibited EPB growth west of similar to 45 degrees W. These electrodynamic changes are consistent with a brief southward followed by prolonged northward interplanetary magnetic field Bz, suggesting overshielding-driven downward drifts.