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

Abstract

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.