Source of the Observed Enhancements in Thermospheric ΣO/N2 During Two Solar Eclipses in 2023

Two solar eclipse events in 2023 appeared to produce considerable enhancements in the thermospheric column density ratio of monatomic oxygen to molecular nitrogen (ΣO/N₂) as measured by TIMED GUVI. We quantify potential sources for eclipse-induced ΣO/N₂ changes and find that the observed enhancements arise from the ionospheric O⁺ radiative recombination contribution to the OI 135.6 nm emission from which ΣO/N₂ is derived. Variations in the solar Extreme Ultra Violet (EUV) and X-ray spectrum, due to the difference between the disk spectrum and the coronal spectrum, are also considered but shown to have negligible contributions to the ΣO/N₂ enhancements. After accounting for the radiative recombination contribution, we constrain the real thermospheric compositional change to the uncertainty level of the measurements of 5%–10%. These results are valuable for the interpretation of eclipse-induced ΣO/N₂ changes that will further first-principle model comparisons and lead to a better understanding of the response of the thermosphere to localized variations in solar EUV and X-ray forcing.