Co-ordinated observations of VHF scintillations in India during February-March 1993

The VHF scintillation observations made at a chain of stations in India during February-March 1993 under AICPITS (third campaign), using the 244 MHz radio beacon from FLEETSAT, and analysed jointly by all participating investigators at a workshop held at Rajkot are described. The occurrence features of scintillations during the third campaign were similar to those observed during the first campaign of March-April 1991. The maximum occurrence was, however, reduced to some extent due to the lower solar activity. The latitudinal variations showed an increased occurrence in the region of about 17-18 degrees latitude. The data during the night of 19-20 Feb. 1993 when an extensive `Ionisation Hole Campaign' was undertaken showed scintillations marked by earlier onset and longer duration at stations Waltair and Nuzvid than at the stations close to the magnetic equator. However, based on the average variations during February-March 1993 the onset at Tiruchendur, Anantpur and Waltair was at nearly the same time. The vertical rise velocity of the plasma depletions, estimated from the time delays in the onset of scintillations at latitudes away from the dip equator, was found to range from 40 m/s to 420 m/s in the altitude region 300-1350 km.