Investigation of lower thermospheric tidal harmonics using ground -based radio and spaceborne optical techniques.

Abstract

Wave like oscillations, with periods of 12 or 24 hours, in the lower thermosphere are characterized using the Sondre Stromfjord incoherent scatter radar (67&deg;N) neutral wind data set spanning twelve Lower Thermosphere Coupling Study (LTCS) campaigns. The effects of different assumptions in the Auto Correlation Function (ACF) fitting program on the data return and quality are investigated. I have used an extensive analysis of radar derived neutral winds to examine the average seasonal structure and variability. Strong semidiurnal oscillations characterize high-latitude lower thermospheric winds at 105 km irrespective of season; however, summer amplitudes are nearly a factor of 2 greater than those observed during winter. Average winter winds appear to have more variability than those deduced for summer. Above 115 km, diurnal oscillations grow and their amplitudes often exceeds typical semidiurnal amplitudes. In general, diurnal zonal and meridional phase profiles suggest an evanescent mode above &sim;110 km with one exception of LTCS-6 measurements which indicate an upward propagating mode. Semidiurnal zonal and meridional phase profiles are indicative of upward propagating tides with wavelengths of &sim;30--50 km. I have demonstrated a technique to facilitate the retrieval of tides from the TIMED-Doppler Interferometer (TIDI) measurements collected on board the Thermosphere-Ionosphere-Mesosphere, Energetics and Dynamics (TIMED) satellite. TIDI sampling schemes will introduce aliasing into the space-time spectrum, complicating the retrieval of tides from the measurements. I have developed a technique for reducing the amount of aliasing in the spectral analysis of TIDI observations, by ingestion of ground-based data into the satellite data set. A multi-dimensional (space-time) least squares fitting approach is applied to the satellite and ground-based data to determine the aliasing spectra. The addition of ground-based data to the TIDI data set reduces the aliased components in the aliasing spectrum. For example, at 57&deg; latitude, the combined ground-based and TIDI data set of a sampled input diurnal (frequency of 1 day<super>-1</super>) signal with zonal wavenumber 1 results in a factor of 4 reduction in the amount of power aliasing into a semidiurnal (frequency of 2 day<super>-1</super>) signal with zonal wavenumber 2. Similarly reductions in the aliased components are observed at other latitudes.