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

Abstract 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/N 2 ) in the Northern Hemisphere. We found that the timing of the O/N 2 equinox transition from winter to summer or vice versa exhibits a progression with latitude, particularly, near spring equinox. The O/N 2 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/N 2 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/N 2 equinox transition is also influenced by the lower atmospheric forcing.