The Source of Tidal Signatures in the TIMED/GUVI O/N2 Ratio

The column number density ratio of atomic oxygen to molecular nitrogen (O/N2 ratio) retrieved from the observations of the TIMED/GUVI has been used as a diagnostic of the variation of the thermospheric neutral composition. Moreover, the TIMED/GUVI O/N2 ratio was recognized recently as a tool for probing the coupling between the thermosphere and atmospheric tides. This study examines the validity of the GUVI O/N2 ratio as a thermospheric parameter and the source of the tidal features in the GUVI O/N2 ratio. The effect of the ionospheric 135.6 nm emission on the GUVI O/N2 ratio retrieval is estimated using the electron density profiles provided by the USU-GAIM model. Our results show that the 135.6 nm emission produced by the equatorial ionization anomaly (EIA) contributes 5~10% to the total 135.6 nm intensity (GUVI observation) and O/N2 ratio. The morphology and seasonal variation of the longitudinal wave pattern in the GUVI 135.6 nm intensity acquired above an altitude of ~300 km show a good agreement with those in the F-region plasma density. However, the longitudinal wave pattern does not appear in the GUVI 135.6 nm intensity below ~300 km altitude. These observations indicate that the ionosphere is the major source of the variation of the GUVI 135.6 nm intensity. The EIA-like feature and longitudinal wave pattern do not appear in the N2 Lyman-Birge-Hopfield band intensity in any altitude. Therefore, the ionospheric 135.6 nm emission is the primary source of the GUVI O/N2 ratio variation in low latitudes and the longitudinal wave feature in the GUVI O/N2 ratio cannot be interpreted as a tidal signature in the thermosphere.