Lower thermospheric neutral winds at Sondre Stromfjord: A seasonal analysis

Data obtained by Sondre Stromfjord incoherent scatter radar during recent Lower Thermospheric Coupling Study (LTCS) campaigns have been analyzed to examine seasonal changes in high-latitude lower thermosphere neutral winds. Data included in this study were obtained with a 2x160 mu s multipulse as well as with higher-resolution multipulse and alternating code techniques and were analyzed with different auto correlation function (ACF) fitting routines. We report neutral wind results obtained during LTCS experiments with the new acquisition and analysis system and compare these results with those acquired using earlier versions of the software. We investigate the effects of different assumptions in the ACF fitting program on the data return and quality. Wind results are intercompared to examine 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. Equinox winds at fall and spring show similar oscillations but are nearly reversed in phase. At 115 km, diurnal oscillations grow and their amplitudes often exceeds typical semidiurnal amplitudes. Tidal amplitudes observed in radar data are compared with the predictions of several models, including National Center for Atmospheric Research thermosphere-ionosphere-general-circulation model (TIGCM) [Roble et al., 1988], global scale wave model (GSWM) [Hagan et al., 1994], and Forbes-Vial model (FV91) [Forbes and Vial, 1991]. These comparisons suggest that theoretical predictions significantly underestimate tidal amplitudes calculated from Sondrestrom incoherent scatter radar measurements.