Spectral Analysis of Variations in Neutral Winds in the Lower Thermosphere Over Thirty Days.

Lower thermospheric neutral wind data obtained with the Millstone Hill incoherent scatter radar during the September 2005 30-day campaign were used to investigate the spectral characteristics of atmospheric tides in the lower thermosphere. Due to problems with aliasing arising from periodic nighttime gaps, the analysis was conducted using the Lomb-Scargle technique for both zonal and meridional winds. Spectral components of 8- hour, 12-hour, 24-hour, quasi-2-day, quasi-5-day, and quasi-10-day waves were observed and found to be localized in various altitude ranges. The semidiurnal component was found to be the strongest spectral feature, followed by the diurnal and the 8 hour waves, while the multi-day components were the weakest components. Several techniques were then employed to verify the authenticity of the spectral peaks in order to determine whether the tides were a result of physical phenomena or a consequence of spectral leakage due to the data truncation and nighttime gaps. Those techniques involved dividing the time series into segments in order to identify shorter period waves, and performing spectral analysis on a box car time series representing the observing program. The latter technique helped to identify how spectral leakage was contributing to the periodograms.