Nmax observed by GOLD (a, b) and NmF2 simulated by WACCM-X (c, d) at 20:10 UT on November 03-04, 2021. The red dotted lines represent the magnetic equator. The black dotted lines represent the 0° longitude. The temporal variations of NmF2 at the 0° longitude (40°S - 40°N latitude) from 9:00 UT and 24:00 UT, simulated by WACCM-X, for November 3rd (e) and 4th (f); The corresponding temporal variations of the peak distance of the South and North EIA crests in (e) and (f), for November 3rd (g) and 4th (h).
JGR-Space Physics: In this study, we conduct an in-depth analysis of Whole Atmosphere Community Climate Model-eXtended (WACCM-X) simulations to examine physical mechanisms of the formation and evolution of an equatorial ionization anomaly (EIA) merging phenomenon during a storm on November 4th, 2021. A quantitative analysis reveals that the rapid decay of the EIA crests at their poleward sides at altitudes of ~200-250 km plays a crucial role in the EIA merging during that day. This rapid decay is due to the fast recombination at low altitudes (~200-250 km) as the plasma are transported downward by the westward disturbance dynamo electric field (DDEF) and poleward neutral winds during the storm. The results suggested EIA-merging is not merely northern and southern EIA crests moving together, but it involves a crucial rapid decay of the EIA crests at their poleward sides that descended to low altitudes (rapid recombination, ~200-250 km), driven by regional electric fields and neutral winds. This study plays a crucial role in our understanding of the evolution and formation of the merged EIA on November 4th, 2021 during the storm.