FIRST SUMMER RESULTS ON WINDS IN THE UPPER MESOSPHERE DERIVED FROM THE 843-NM HYDROXYL EMISSIONS MEASURED FROM THE BEAR-LAKE-OBSERVATORY, UTAH

The Imaging Fabry-Perot Interferometer (IFPI) at the Bear Lake Observa tory (BLO), Utah (41.9 degrees N, 111.4 degrees W) is used for studies of the aeronomy of the middle and upper atmosphere. Wind and temperat ure structure can be determined from observations of the Doppler shift and Doppler broadening of the airglow and auroral emissions from the mesosphere and thermosphere. The mesospheric winds recorded at the end of August, September and early October 1992 are consistent with a sem i-diurnal tidal variation. The amplitude of this variation is approxim ately 30 ms(-1) at the end of August and early September and approxima tely 20 ms(-1) at the end of September and early October. However, dur ing June and July, the semi-diurnal tidal variation, if present, is we ak, with amplitude < 5 ms(-1). No consistent semidiurnal tidal variati on is observed during late October 1992. During the solstice period, a ntisymmetric tidal components may be preferentially generated in such a way that they can result in destructive interference with the normal ly dominant symmetric modes, resulting in a decrease of tidal variatio n. This is consistent with the observed decrease in tides during the J une, July and late October periods. Near the equinoxes, however, the e xcitation of these antisymmetric modes is expected to be weaker, possi bly explaining why a pronounced and consistent semi-diurnal tidal Vari ation has been observed during the August, September and early October periods. In contrast, the mesospheric winds derived from the Sheffiel d Meteor Wind Radar (53.4 degrees N, 1.5 degrees W) reveal a clear sem i-diurnal tidal variation throughout the year, with an amplitude that may vary between 15 ms(-1) and 50 ms(-1), being about 25 ms(-1) on ave rage. The IFPI records winds from a region of the atmosphere centred a t 87 km, whereas the Sheffield Meteor Wind Radar measures winds centre d at 95 km. Therefore, the two regions may experience different tidal modes due to the different latitude, longitude and altitude of the obs erved regions and/or the different topography of the observing sites. Some proposed reasons for these differences are presented.