Position of the mid-latitude trough in the topside ionosphere as deduced from ESRO 4 observations

The position of the mid-latitude trough in the topside ionosphere is determined from electron density data of ESRO 4 during quiet solar conditions. An analysis of about 300 trough observations in the northern and southern hemispheres showed that the trough was generally seen only during the night from 19 to 05 hr LT. During this period, the invariant latitude of the minimum of the trough was found to decrease both with increasing geomagnetic disturbance represented by Kp and with increasing LT after sunset.