Equinox transition in wind and airglow observations

The equinox transition marks two significant intervals in the annual variability of the mesosphere and lower thermosphere. A scenario of this phenomenon is emerging in which for atomic oxygen, the winter period is characterized by downward flow, leading to enhanced oxygen concentrations and high levels of oxygen airglow. These high winter airglow levels are punctuated by large fluctuations associated with the highly variable winter dynamics, including stratospheric warmings, superposed on a monotonic downward trend. The summer is a time of less variability and weaker airglow emission associated with atmospheric upwelling, ending with a buildup of atomic oxygen in the fall. Within this general picture the observations depend significantly on latitude. At high latitudes the effects of upwelling and downwelling are clear and these persist to latitudes as low as 40degrees, but at still lower mid-latitudes a semi-annual variation with equinoctial maxima seems to dominate, as observed by earlier workers. This scenario is supported by the ground-based and satellite observations presented here. The dynamical coupling with the airglow is explored by comparing airglow observations at Stockholm (59.5degreesN, 18.2degreesE) with the zonal wind observations at nearby Juliusruh (54.6degreesN, 13.4degreesE), while WINDII data are used to connect the two as well as providing mid-latitude data. In fitting these equinox transition observations into the annual variation these observations are compared with predictions from the TIME-GCM model, with good overall agreement. (C) 2004 Elsevier Ltd. All rights reserved.