WINDII observations of thermospheric O(1D) nightglow emission rates, temperature, and wind: 1. The northern hemisphere midnight temperature maximum and the wave 4

Abstract

AbstractThe midnight temperature maximum (MTM) is a large‐scale neutral temperature anomaly with a wide‐ranging effect on the nighttime thermospheric dynamics at low latitudes. The focus of the current study is an investigation of the extent of the MTM to northern midlatitudes (20°N–40°N), employing multiyear observations of O(1D) airglow volume emission rates (VERs), Doppler temperatures (DoT), and neutral winds over the altitude range of 190–300 km by the Wind Imaging Interferometer (WINDII) experiment on board the Upper Atmosphere Research Satellite. The MTM dependence on longitude, season, local time, and altitude was examined. Midnight maxima were observed both in the O(1D) VER and DoT with peaks at ~24 local time (LT) during winter solstice, 22 LT for fall equinox, and 2 LT for spring equinox. The peak in the DoTs was marked with strong southward meridional winds (e.g., ~100–150 m s−1). Latitude/longitude maps of the VER and DoT revealed wave 4 signatures most persistently seen around local midnight, with very little variation in phase, while the amplitude of the individual peaks varied with time. The observed perturbations in the O(1D) VER and temperature were out of phase with respect to longitude. Two of the peaks at ~100°E and 260°E–300°E were almost stationary, while the other two peaks varied in strength over the period of observation. A common feature was that one of the wave 4 peaks was always over the American sector; it was constant with local time, and the meridional wind was southward only over the same region.