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.