Equinox transitions of thermosphere O/N2 and meridional circulation in the Northern Hemisphere as observed by NASA's GOLD and ICON missions

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.