Nightglow zenith emission rate variations in O(1S) at low latitudes from wind imaging interferometer (WINDII) observations

Abstract

Volume emission rate data of the O(1S) nightglow layer measured by the wind imaging interferometer (WINDII) aboard the Upper Atmosphere Research Satellite (UARS) are vertically integrated to give equivalent zenith column emission rates. The data are restricted to tropical latitudes and categorized according to season. The migrating tidal signature is extracted by plotting zonally averaged data as a function of local time. The main features are a premidnight maximum at the equator during equinox intervals and a midnight maximum at 20°N and S. The two equinoxes are not equivalent, nor is the behavior identical in the two hemispheres. Winter periods show little or no well-defined tidal signature. Longitudinal variations at fixed local time were also studied. These variations are larger than the tidal signature and are geographically random, showing no consistent identification with land masses. Some of these variations could be of planetary scale, while others are probably long-wavelength gravity waves, but in any case they were seen to extend over 10° of latitude at least. Zenithal emission rate measurements exceeding 500 R occurred at the coincidence of these longitudinal wave structures and the tidal maxima. The suggestion is put forward that much of the perceived tidal variability in the tropical lower thermosphere may be explained by such wave-tide superpositions.