Zonal drifts of ionospheric irregularities at temperate latitude in the Indian region

The systematic time differences observed in the onset of postsunset VHF scintillations recorded simultaneously at Ujjain (Geogr. lat. 23.2^○N, Geogr. long. 75.6^○E) and Bhopal (Geogr. lat. 23.2^○N, Geogr. long. 77.6^○E), situated at the peak of the anomaly crest in the Indian region, have been analysed to determine the zonal drifts of scintillation-producing irregularities. The method is based on the assumption that the horizontal movement of irregularities does not change while crossing the F-region cross-over points of these stations. The calculated velocities of irregularities indicate an eastward drift decreasing from about 180 m s⁻¹ to 55 m s⁻¹ during the course of night. In the premidnight period, the drifts are reduced under the magnetically disturbed conditions. The average east-west extension of irregularites is found to be in the range of 200–500 km.