Formation Mechanism and Interhemispheric Asymmetry of Storm‐Enhanced Density During April 2023 Storm Revealed by GITM

We investigate the formation and interhemispheric asymmetries of storm‐enhanced density (SED) during the April 23–24, 2023 geomagnetic storm using the Global Ionosphere‐Thermosphere Model (GITM) coupled with high‐latitude drivers from the Geospace model. This storm, driven by an interplanetary coronal mass ejection (ICME), featured two southward IMF intervals—the ICME sheath followed by a magnetic cloud (MC)—each producing SED with distinct asymmetries. During the sheath phase, SEDs formed in both hemispheres with a stronger isolated peak in the south. During the MC phase, a weaker SED appeared only in the north, despite a stronger IMF B z . GITM results indicate that equatorward neutral wind‐induced upward ion drift was the primary driver of midlatitude SEDs, while convection electric fields dominated high‐latitude SED and plume lifting. Differences in SED response between the two phases highlight the role of thermospheric composition and its temporal history in shaping storm‐time ionospheric density, including pre‐storm modulation, storm‐time reinforcement, and interhemispheric asymmetry of magnetic poles with universal time (UT) effect. The prominent southern SED during the sheath phase was attributed to a higher O/N2 ratio from preconditioning and storm‐time reinforcement, whereas the expanded convection during the MC phase lowered O/N2 in southern midlatitudes and limited SED growth. The impacts of large‐scale traveling atmospheric/ionospheric disturbances (TADs/TIDs) on SED formation were minimal. This study quantifies various factors driving SED formation and highlights the sustained impact of ICME sheath during a double‐dip storm on ionosphere‐thermosphere dynamics and the role of thermospheric composition in modulating SED location, characteristics, and hemispheric asymmetry.