Investigation of Interhemispheric Asymmetry of the Thermospheric Composition Observed by GOLD During the First Strong Geomagnetic Storm in Solar-Cycle 25, 1: IMF B_y Effects

An interhemispheric asymmetry of thermospheric Oxygen (O) to Nitrogen (N₂) column density ratio (∑O/N₂) variations was observed by the Global-scale Observations of the Limb and Disk (GOLD) during the 3–5 November 2021 geomagnetic storm. ∑O/N₂ depletion is more equatorward in the local morning than in the local afternoon in the northern hemisphere (NH), while more equatorward in the local afternoon than the local morning in the southern hemisphere (SH). The Thermosphere Ionosphere Electrodynamics General Circulation Model simulations reproduced the observed ∑O/N₂ asymmetry. The impacts of interplanetary magnetic field (IMF) east-west component (B_y) on the interhemispheric asymmetry of ∑O/N₂ depletion was investigated. Comparisons of simulation results with and without real B_y show that the dominant positive IMF B_y results in the Joule heating maximum occurring in the local afternoon of NH and local morning in the SH during the storm initial phase, which generates the corresponding ∑O/N₂ changes. Additionally, the IMF B_y during this storm are more crucial in the SH than in the NH. Without B_y, equatorward penetration of the ∑O/N₂ depletion in GOLD Field of View are almost same in the SH. However, in the NH, there is still more equatorward depletion in the local morning than the local afternoon without B_y.