A simulation study of thermospheric neutral winds over the MU radar

The climatological thermospheric winds observed by the Shigaraki middle and upper atmosphere (MU) radar in East Asia are compared for the first time with simulation results from the National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General Circulation Model (NCAR-TIEGCM). The comparisons show that the predicted meridional winds are in excellent agreement with the observations; their differences generally fall within 20 m/s except for the summer nighttime at high solar activity. A harmonic analysis of the observed and model meridional winds is carried out to extract the mean winds and the amplitudes and phases of diurnal, semidiurnal, and terdiurnal components. The model tidal waves are also in good agreement with the observed values except for the summer diurnal amplitude at high solar activity. Comparisons of the model electron densities with the radar observations suggest that the solar activity variation of the meridional winds can be mainly explained by the role of ion drag. Interestingly, the numerical simulations predict a feature not previously interpreted well: the diurnal amplitude and the mean wind depend nonlinearly on solar activity because the electron density saturates at high solar activity levels. It is further shown that both the observed and predicted diurnal phases remain almost stable at different solar activity levels, regardless of season.