NOCTURNAL OBSERVATIONS OF THE SEMIDIURNAL TIDE AT A MIDLATITUDE SITE

Fabry-Perot interferometer observations of the mesospheric hydroxyl em ission and the lower thermospheric O I (5577 Angstrom) emission have b een conducted from an airglow observatory at a dark field site in sout heastern Michigan for the past several years. The primary functions of the observatory are to provide a database for correlative observation s with the UARS satellite and to provide a synoptic measurement progra m for the coupling energetics and dynamics of atmospheric regions effo rt. An intensive operational effort between May 1993 and July 1994 has resulted in a substantial data set from which neutral winds have been determined from the bifilter acquisition sequence. A ''best fit'' ana lysis in the least squares sense of the simultaneous measurements of t he neutral winds to a 12-hour periodicity has provided amplitude and p hase parameters for the semidiurnal tide as well as a measure of the m ean wind. The measured tidal amplitude is greater at the higher altitu de, though the seasonal behavior at both altitudes is similar with gre ater amplitudes during August/September and April/May. Both meridional and zonal wind components are consistent with a semidiurnal tidal des cription during the entire observational sequence except for the May t o July 1993 period. The mean winds show annual variation in the meridi onal flow, being equatorward from May to October and poleward during t he winter. The zonal flow is primarily eastward during the entire obse rvational window with higher speed flows during May/June at the higher altitude and June/July at the lower altitude. A comparison with a sem idiurnal tidal model indicates that the measured tidal amplitudes are a factor of 2 times greater, while the phases show similar equinoctial transitions.