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