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 Observatory (BLO), Utah (41.9°N, 111.4°W) is used for studies of the aeronomy of the middle and upper atmosphere. Wind and temperature 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 semi-diurnal tidal variation. The amplitude of this variation is approximately 30 ms−1 at the end of August and early September and approximately 20 ms−1 at the end of September and early October. However, during June and July, the semi-diurnal tidal variation, if present, is weak, with amplitude \u3c 5 ms−1. No consistent semi-diurnal tidal variation is observed during late October 1992. During the solstice period, antisymmetric tidal components may be preferentially generated in such a way that they can result in destructive interference with the normally dominant symmetric modes, resulting in a decrease of tidal variation. This is consistent with the observed decrease in tides during the June, July and late October periods. Near the equinoxes, however, the excitation of these antisymmetric modes is expected to be weaker, possibly explaining why a pronounced and consistent semi-diurnal tidal variation has been observed during the August, September and early October periods. In contrast, the mesospheric winds derived from the Sheffield Meteor Wind Radar (53.4°N, 1.5°W) reveal a clear semi-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 average. The IFPI records winds from a region of the atmosphere centred at 87 km, whereas the Sheffield Meteor Wind Radar measures winds centred at 95 km. Therefore, the two regions may experience different tidal modes due to the different latitude, longitude and altitude of the observed regions and/or the different topography of the observing sites. Some proposed reasons for these differences are presented