The effect of the 135.6 nm emission originated from the ionosphere on the TIMED/GUVI O/N₂ ratio

The column number density ratio of atomic oxygen to molecular nitrogen (O/N₂ ratio) provided by the Global Ultraviolet Imager (GUVI) onboard the Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED) satellite has been used as a diagnostic of the thermospheric neutral composition. However, a recent study claimed that the GUVI O/N2 ratio is not a pure thermospheric parameter in low latitudes during periods of low geomagnetic activity. This study quantifies the O/N₂ ratio contamination by the ionosphere using the GUVI observations and model ionosphere acquired from 31 August to 2 September 2002. During this period, the local time of the GUVI observation was near 1500 and the average Kp index was 2°. The 135.6 nm emission originated from the ionosphere is estimated using the electron density profiles provided by the Utah State University-Global Assimilation of Ionospheric Measurements model. Our results show that the 135.6 nm emission originated from the equatorial ionization anomaly (EIA) contributes 5-10% to the total 135.6 nm intensity and O/N₂ ratio. The EIA feature and longitudinal wave patterns in the GUVI 135.6 nm intensity maps are identified above an altitude of 300km and show a good agreement with those in the F region plasma density. However, the EIA feature and longitudinal wave patterns do not appear in the GUVI 135.6 nm intensity maps below an altitude of 300km and in the GUVI N₂ Lyman-Birge-Hopfield band intensity maps in any altitude. These observations indicate that the longitudinal wave patterns in the GUVI O/N₂ ratio represent the ionospheric phenomenon.