Troughs in the subauroral ionosphere (L~4) first were clearly identified from ionograms obtained with the topside sounder satellite Alouette 1 [Muldrew, 1965> and since have been the subject of considerable study. These studies have shown that the main ''midlatitude trough'' is a feature that forms in the nighttime ionosphere extending room ~20 hrs LT to ~06 hrs LT in which the F-region density decreases (typically by about a factor of five) in an interval of magnetic latitude of ~5¿. The trough minimum magnetic latitude of ~5¿. The trough minimum typically is found near &Lgr;=65¿ in the evening hours but moves towards 8, there were instances when they were observed as low at L~4 at midday; these invariably were disturbed ocasions (Kp>+3). It often is difficult to extract separately the local time and UT dependence of ionospheric features from satellite data and the differential-Doppler results reported here permitted such a separation.