First Results From the Retrieved Column O/N₂ Ratio From the Ionospheric Connection Explorer (ICON): Evidence of the Impacts of Nonmigrating Tides

Abstract

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.

Reducing the ionospheric contamination effects on the column O/N2 ratio and its application to the identification of non-migrating tides

“…The modeled amplitudes seem to underestimate the total wavenumber-3 and wavenumber-4 amplitudes from the ICON analysis in Figure 4 but do match the dominant tide apparent in the observations. This tendency to underestimate the tidal amplitude in ΣO/N 2 is consistent with the earlier findings of England et al (2021), who did not correct the ICON SW data for radiative recombination. Figure 6 summarizes the relevant tides from HME-ICON output.…”

“…3. Observed amplitudes are higher than those modeled by TIEGCM-ICON in agreement with the earlier study by England et al (2021).…”

“…This work uses Version 4 of ΣO/N 2 data and Version 3 of the calibrated farultraviolet radiances, which were the latest available when this work was undertaken. In addition, this work reassesses part of the analysis of England et al (2021) with the same ΣO/N 2 data version used therein. Because the focus of this paper is a method to remove the ionospheric contribution from the retrieval of ΣO/N 2 and its implications for the identification of upward propagating nonmigrating tides, the actual version of radiances used is not of primary relevance; data version is accounted for with our calibration method (Section 3).…”

“…It has been hypothesized that non-migrating tides substantially impact the ΣO/N 2 ratio. Atmospheric theory predicts that tides perturb ΣO/N 2 as a result of the species-dependent impact of the vertical tidal winds (Cui et al, 2014;England et al, 2021). Indeed, these fluctuations appear in physics-based models of the thermosphere (England et al, 2010;Jones et al, 2014;Roble and Shepherd, 1997).…”

“…Observations of the ΣO/N 2 ratio from the Far Ultra-violet Imager (FUV) onboard the Ionospheric Connection Explorer (ICON; provide an opportunity to study non-migrating tides in thermospheric composition (England et al, 2021). The rapid local time precession afforded by the ICON observatory in a 27°inclined orbit is a distinct advantage because seasonal variations do not strongly alias into the tidal signal during the time required to sample all local solar times (~27 days) .…”