Equinox Transitions of Thermosphere O/N₂ and Meridional Circulation in the Northern Hemisphere as Observed by NASA's GOLD and ICON Missions

Leveraging the unique perspective enabled by Global-scale Observations of the Limb and Disk, we examined the characteristics of equinox transitions in the thermospheric column integrated ratio of atomic oxygen to molecular nitrogen (O/N₂) in the Northern Hemisphere. We found that the timing of the O/N₂ equinox transition from winter to summer or vice versa exhibits a progression with latitude, particularly, near spring equinox. The O/N₂ equinox transition is far slower during spring compared to fall, leading to a remarkable seasonal asymmetry. Ionospheric Connection Explorer observed a prominent asymmetry in the summer-to-winter circulation in the middle to upper thermosphere, implying that the inter-hemispheric circulation plays a crucial role in the O/N₂ equinox transition. Additionally, since the wave-driven meridional circulation in the lower thermosphere displays a seasonal asymmetry between the northward-to-southward and southward-to-northward transitions, we would anticipate that the O/N₂ equinox transition is also influenced by the lower atmospheric forcing.