An Investigation of the Local Time Behavior of ΣO/N₂ During Solstice

AbstractThe thermospheric column O/N2 ratio (ΣO/N2) exhibits complex spatial and temporal variations and is a key parameter in diagnosing the state of the thermosphere and ionosphere. The solar cycle, seasonal, latitudinal and longitudinal variations of ΣO/N2 have been studied in the past few decades. However, the local time variation of ΣO/N2 is rarely investigated. At solstice, the most important feature of ΣO/N2 is the well‐known summer‐winter difference caused by the global meridional circulation. Based on TIMED/GUVI observations from 2002 to 2022, it was found that the daytime pattern of ΣO/N2 exhibits significant hemispheric differences superimposed on the more prominent seasonal distribution. ΣO/N2 decreases soon after sunrise in the winter hemisphere, while it occurs much later in the summer hemisphere. This hemispheric difference in the local time variation of ΣO/N2 is generally reproduced by the empirical model MSIS and the physics‐based model TIEGCM. And the TIEGCM simulation results suggest that it is mainly attributed to the meridional advection. In the summer hemisphere, upward winds which decrease ΣO/N2 are weak in the early morning and compete with the enhancement of ΣO/N2 caused by the meridional advection, resulting in a later turning time around 11:00 LT. In the winter hemisphere, the reduction of ΣO/N2 caused by daytime upward winds is superimposed on the reduction of ΣO/N2 induced by the meridional advection, resulting in an earlier turning time around 7:00 LT. Additionally, ΣO/N2 peaks much later in regions near the magnetic pole than those far from the magnetic pole in the summer hemisphere.