The temperature and composition of the middle thermosphere change drastically with altitude. The neutral temperature increases sharply with altitude while the density of neutral constituents tends to decrease exponentially according to their respective scale heights (as well as the production and loss mechanisms for certain species). Atomic oxygen and molecular nitrogen are the two main constituents of this region. The vertical column density ratio of these two ( E  O/N 2 ) is a sensitive measure of thermosphere composition. Any upward propagating waves present in the mesosphere/lower thermosphere (MLT) can impact middle thermosphere temperature and composition structures. A subset of thermal atmospheric tides, including some

In near‐Earth space, variations in thermospheric composition have important implications for thermosphere‐ionosphere coupling. The ratio of O to N2 is often measured using far‐UV airglow observations. Taking such airglow observations from space, looking below the Earth's limb allows for the total column of O and N2 in the ionosphere to be determined. While these observations have enabled many previous studies, determining the impact of nonmigrating tides on thermospheric composition has proved difficult, owing to a small contamination of the signal by recombination of ionospheric O+. New ICON observations of far‐UV are presented here, and their general characteristics are shown. Using these, along with other observations and a global circulation model, we show that during the morning hours and at latitudes away from the peak of the equatorial ionospheric anomaly, the impact of nonmigrating tides on thermospheric composition can be observed. During March–April 2020, the column O/N2 ratio was seen to vary by 3–4% of the zonal mean. By comparing the amplitude of the variation observed with that in the model, both the utility of these observations and a pathway to enable future studies is shown.

Prior investigations have attempted to characterize the longitudinal variability of the column number density ratio of atomic oxygen to molecular nitrogen (ΣO/N2) in the context of non-migrating tides. The retrieval of thermospheric ΣO/N2 from far ultra-violet (FUV) emissions assumes production is due to photoelectron impact excitation on O and N2. Consequently, efforts to characterize the tidal variability in O/N2 have been limited by ionospheric contamination from O+ radiative recombination at afternoon local times (LT) around the equatorial ionization anomaly. The retrieval of ΣO/N2 from FUV observations by the Ionospheric Connection Explorer (ICON) provides an opportunity to address this limitation. In this work, we derive modified ΣO/N2 datasets to delineate the response of thermospheric composition to non-migrating tides as a function of LT in the absence of ionospheric contamination. We assess estimates of the ionospheric contribution to 135.6 nm emission intensities based on either Global Ionospheric Specification (GIS) electron density, International Reference Ionosphere (IRI) model output, or observations from the Extreme Ultra-Violet imager (EUV) onboard ICON during March and September equinox conditions in 2020. Our approach accounts for any biases between the ionospheric and airglow datasets. We found that the ICON-FUV dataset, corrected for ionospheric contamination based on GIS, uncovered a previously obscured diurnal eastward wavenumber 2 tide in a longitudinal wavenumber 3 pattern at March equinox in 2020. This finding demonstrates not only the necessity of correcting for ionospheric contamination of the FUV signals but also the utility of using GIS for the correction. Hosted fileessoar.10512866.1.docx available at https://authorea.com/users/557195/articles/611055reducing-the-ionospheric-contamination-effects-on-the-column-o-n2-ratio-and-itsapplication-to-the-identification-of-non-migrating-tides Hosted file supporting_information.docx available at https://authorea.com/users/557195/articles/611055reducing-the-ionospheric-contamination-effects-on-the-column-o-n2-ratio-and-itsapplication-to-the-identification-of-non-migrating-tidesReducing the ionospheric contamination effects on the column O/N 2 ratio and its application to the identification of non-migrating tides

Prior investigations have attempted to characterize the longitudinal variability of the column number density ratio of atomic oxygen to molecular nitrogen (normalΣ ${\Sigma }$O/N2) in the context of non‐migrating tides. The retrieval of thermospheric normalΣ ${\Sigma }$O/N2 from far ultra‐violet (FUV) emissions assumes production is due to photoelectron impact excitation on O and N2. Consequently, efforts to characterize the tidal variability in normalΣ ${\Sigma }$O/N2 have been limited by ionospheric contamination from O+ + e radiative recombination at afternoon local times (LT) around the equatorial ionization anomaly. The retrieval of normalΣ ${\Sigma }$O/N2 from FUV observations by the Ionospheric Connection Explorer (ICON) provides an opportunity to address this limitation. In this work, we derive modified normalΣ ${\Sigma }$O/N2 datasets to delineate the response of thermospheric composition to non‐migrating tides as a function of LT in the absence of ionospheric contamination. We assess estimates of the ionospheric contribution to 135.6 nm emission intensities based on either Global Ionospheric Specification (GIS) electron density, International Reference Ionosphere (IRI) model output, or observations from the Extreme Ultra‐Violet imager (EUV) onboard ICON during March and September equinox conditions in 2020. Our approach accounts for any biases between the ionospheric and airglow datasets. We found that the ICON‐FUV data set, corrected for ionospheric contamination based on GIS, uncovered a previously obscured diurnal eastward wavenumber 2 tide in a longitudinal wavenumber 3 pattern at March equinox in 2020. This finding demonstrates not only the necessity of correcting for ionospheric contamination of the FUV signals but also the utility of using GIS for the correction.

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