Observations of high-latitude lower thermospheric winds from Thule Air Base and Søndre Strømfjord, Greenland

Lower thermospheric winds have been determined from Fabry-Perot interferometer (FPI) measurements of the Doppler shift of the 5577-Å O(₁S) emission over Thule Air Base (76.5°N, 69.0°W) and Søndre Strømfjord (67.0°N, 50.9°W), Greenland. These winds normally correspond to the altitude of the peak of the airglow O(₁S) emission layer, near 97 km. The altitude ambiguity due to auroral contamination has been reduced by eliminating data when the intensity of the emission increases significantly. Contamination by airglow emission of 5577-Å O(¹S) originating from higher altitudes has been investigated by an FPI simulation code. The simulation results indicate that this latter emission may contribute an anomalous diurnal oscillation to ground-based 5577-Å O(¹S) FPI measurements of lower thermospheric wind. The agreement of diurnal phases between that deduced from the green-line measurements and that determined from simultaneous red-line observation supports this conclusion. The same simulation applied to observations from Søndre Strømfjord shows that the upper layer contamination is much weaker and is not serious. Significant day-to-day variation is evident in the lower thermospheric wind field. Average neutral winds are calculated, and a harmonic analysis is carried out to examine the major low-frequency wind components. The seasonal variations of these wind components are compared with radar data and model predictions. The observations are generally in good agreement with model results. The comparison between FPI and radar results also shows reasonable agreement. The semidiurnal amplitudes observed with the Søndre Strømfjord radar during the Lower Thermospheric Coupling Study (LTCS-1) and LTCS-2 periods are always greater than the climatological values obtained from averaging FPI and Chatanika radar observations. This result shows the variability that can be expected when comparing "instantaneous" estimates of tidal parameters with climatological results.