COORDINATED INCOHERENT-SCATTER RADAR OBSERVATIONS OF THE SEMIDIURNAL TIDE IN THE LOWER THERMOSPHERE

The incoherent scatter radars at Arecibo (18.3-degrees-N), Millstone Hill (42.6-degrees-N), and Sondrestrom (67-degrees-N) simultaneously observed the thermal structure and dynamics of the Earth's lower thermosphere during the period September 21-25, 1987, as part of the Lower Thermosphere Coupling Study. Plasma spectral measurements were used to derive ion temperatures and horizontal neutral wind velocities in the altitude region 90-140 km, and semidiurnal harmonics were determined from the data sets. The daily mean temperatures were found to be within 40-degrees-K at the three stations, and the largest semidiurnal temperature amplitudes, exceeding 70-degrees-K above 115 km, were recorded at Arecibo. Mean winds were about +/- 30 m/s at most altitudes, shifting from eastward to westward directions at 115 km, and the semidiurnal amplitudes were in the range 20-70 m/s. Phase variations with height reveal reasonable consistency among the three stations, within 3-4 hours, and indicate an average vertical wavelength of 40-70 km. However, a strong variation is seen in the vertical structure in the 110- to 130-km region, suggesting the presence of different tidal modes. Comparison with the most current tidal propagation model by Forbes and Vial (this issue) indicates that the model tends to underestimate the amplitudes for this time period, particularly above 110 km. By virtue of the latitudinal distribution of the three radars their coordinated observations of the lower thermosphere serve to constrain numerical models and improve our understanding of this important yet complex region of the atmosphere.