The X-Pattern Merging of the Equatorial Ionization Anomaly Crests During Geomagnetic Quiet Time

A unique phenomenon—A geomagnetically quiet time merging of Equatorial Ionization Anomaly (EIA) crests, leading to an X-pattern (EIA-X) around the magnetic equator—has been observed in the night-time ionospheric measurements by the Global-scale Observations of the Limb and Disk mission. The pattern is also reproduced in an ionospheric model that assimilates slant Total Electron Content from Global Navigation Satellite System and Constellation Observing System for Meteorology, Ionosphere, and Climate 2. A free-running whole atmospheric general circulation model simulation reproduces a similar pattern. Due to the similarity between measurements and simulations, the latter is used to diagnose this heretofore unexplained phenomenon. The simulation shows that the EIA-X can occur during geomagnetically quiet conditions and in the afternoon to evening sector at a longitude where the vertical drift is downward. The downward vertical drift is a necessary but not sufficient condition. The simulation was performed under constant low-solar and quiescent-geomagnetic forcing conditions, therefore we conclude that EIA-X can be driven by lower-atmospheric forcing.