PROPAGATION OF THE SOLAR SEMIDIURNAL TIDE IN THE MESOSPHERE AND LOWER THERMOSPHERE AT MID LATITUDES

The meteor wind radar at Durham and the incoherent scatter radar at Millstone Hill, which are closely located at 43-degrees-N, 71-degrees-W, were used simultaneously during the first campaign of the Lower Thermosphere Coupling Study in September 1987 to map the structure of the semidiurnal tidal wind from 80 km to 135 km. The results from the two techniques are consistent and indicate overall continuity in the observed propagation of the semidiurnal tide in both amplitude and phase. Northward winds gradually increase from about 30 m/s at 80 km to 60 m/s above 110 km, while the eastward winds are near 20 m/s below 100 km and reach a maximum of 70 m/s at 115 km. Phase variations indicate a vertical wavelength of the propagating tidal mode of about 100 km at all altitudes for the eastward component, but the incoherent scatter results suggest a shorter wavelength for the northward component above 100 km. The results obtained during the campaign are similar to climatological averages obtained from nonsimultaneous data sets from the two radars. Comparison with numerical model results from Forbes and Vial (this issue) indicates that the model underestimates the wind amplitudes and reaches the maximum value about 10 km below the observed peak; the phase variations lag the observations by 2 hours but indicate an overall structure with altitude that is very similar to the data.