HF Doppler observations of medium-scale travelling ionospheric disturbances at mid-latitudes

From 1972 to 1975 F-region medium-scale travelling ionospheric disturbances (MSTIDs) were observed at Leicester, U.K. (52 deg 32 min N 1 deg 8 min W) by means of the HF Doppler technique. Most of the features of the disturbances previously reported in the literature are confirmed, with the exception of the apparent seasonal variation in the propagation direction. The measured wave azimuth rotates clockwise through 360 deg in 24 h, supporting theoretical predictions concerning the filtering effect of the neutral wind in the Northern Hemisphere. The most commonly observed direction of wave propagation, however, is displaced from the antiwind direction and is located at an azimuth of 130-140 deg relative to the wind. A periodic variation of the direction of wave propagation with respect to the antiwind direction is evident, which may indicate that lower atmospheric winds can have greater influence on waves at the thermospheric heights than previously supposed. A synoptic survey of the data set reveals little correlation between wave occurrence and auroral processes, and it is unlikely that high-latitude sources are responsible for many of the MSTIDs observed at midlatitudes.