The origin of the nonmigrating tidal structure in the column number density ratio of atomic oxygen to molecular nitrogen

Abstract

[1] The longitudinal modulation of the column number density ratio of atomic oxygen to molecular nitrogen (O/N2 ratio) is interpreted as a tidal structure in the thermospheric neutral composition. However, the close similarity between the tidal structures in the ionosphere and O/N2 ratio raises a question regarding this interpretation. We examine the origin of the tidal structure in the O/N2 ratio by using the thermospheric limb data for 2003 provided by the Global Ultraviolet Imager onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. Our results show that the longitudinal variation of the O/N2 ratio at the location of the equatorial ionization anomaly (EIA) is accounted for by the 135.6 nm emission produced by the EIA. Because the O/N2 ratio derived from optical observations is contaminated by the radiative O+ recombination, the O/N2 ratio is not a useful indicator of the tidal structure in the thermosphere.