Measurements of the neutral temperature in the altitude region 100 to 130 km during the past several years at Millstone Hill (42°N) and St. Santin (45°N) have revealed that, on the average, there exist strong semidiurnal tidal effects which can be identified with the (2,4) tidal mode propagating from the stratomesosphere. The average amplitude of the temperature oscillation reaches a maximum of about 35 K at 116 km with maxima at 0630 and 1830 hr LT. At each station, large variability is found in amplitude and phase from one observing period to another, particularly during winter. From classical tidal theory, the average temperature behavior implies a semidiurnal zonal wind oscillation with an amplitude of about 45 m sec−1 at 110 km having eastward maxima at about 0500 and 1700 hr LT, and a meridional component with similar amplitude having southward maxima at 0800 and 2000 hr LT. The downward phase velocity is about 4 km hr−1. These results are in good agreement with direct wind measurements at St. Santin, and with some recently reported characteristics of the height variations of sporadic-E at midlatitudes.
At Arecibo (18°N), semidiurnal oscillations have also been found to dominate the E-region temperature and wind structure with similar amplitude and phase velocities as above; however, the observed phase behavior is more variable than at midlatitudes and the average results do not allow reliable mode identification. Some results of recent simultaneous observations at low and midlatitudes are presented which imply that during any particular observing period, the E-region structure must be controlled by local effects.