Longitudinal and seasonal variations of O(1D) nightglow emission maxima at southern midlatitudes

The Wind Imaging Interferometer (WINDII) experiment on the Upper Atmosphere Research Satellite (UARS) offers a global view of the O(1D) airglow emission rates and neutral winds over the entire altitude range from 190 to 300 km, and provides a unique means for drawing a self-consistent picture of the state of the nighttime upper thermosphere at southern mid-latitudes and by proxy–the ionosphere. The focus of the current study is an investigation of the global seasonal, local time, altitude and longitudinal variability of O(1D) nightglow at southern mid-latitudes (20°S–40°S) employing multi-year observations of O(1D) airglow VERs and neutral winds over the same altitude range and by the same instrument. The results reported here are from four austral seasons, two summers (December solstice, 1992–1993 and 1994–1995), and two equinoxes (March and September 1992). A clear midnight O(1D) VER enhancement is observed at high solar activity during austral summer solstice and equinox (DJF, 1992–1993, and SON, 1992) and at low solar activity during summer (DJF, 1994–1995). The midnight VER enhancement was followed by a wave 4 signature developing after local midnight during all seasons considered particularly during high solar activity summer solstice and spring equinox. The analysis also revealed a complex global O(1D) nightglow VER field, which independently of season exhibited vast regions of very low O(1D) VER (1–4 photons cm−3s−1) in the pre-midnight period at 200°E–300°E longitude throughout the entire latitude range of 5°S–40°S. Particularly noticeable was a region of very low VERs observed poleward of 25°S throughout the local night independent of local time and seasons, over the longitude range from 340°E to 100°E–120°E, which was co-located with the midlatitude ionospheric trough. Another persistent signature observed was an enhancement at 100°E–200°E, at latitudes south of 25°S–30°S, which corresponds to a wave 1 or is a part of a wave 2 signature, likely the result of transport of plasma associated with the Weddell Sea Anomaly. The most prominent result revealed by the study is the role of the wave 4 and wave 1 in the coupling of the lower atmosphere and the thermosphere/ionosphere.