COORDINATED MULTISTATION VHF SCINTILLATION OBSERVATIONS IN INDIA DURING MARCH-APRIL 1991

A number of VHF scintillation stations are currently operating in Indi a as part of the All India Co-ordinated Programme of Ionospheric and T hermospheric Studies (AICPITS). The chain covers latitudes right from the magnetic equator to beyond the anomaly crest region. During March- April 1991 co-ordinated observations were made at all the stations of the chain and the data collected analysed jointly. The quarter-hourly values of the occurrence of scintillations along with the start and en d times of the patches of scintillations during each night form the da ta base. Nocturnal variations of the percentage occurrence of scintill ations, histograms of percentage occurrence of the number of patches o f scintillations during the course of a night and of the patch duratio n have been computed for each station. Scintillations generally start between 1930 and 2000 hrsIST. The stations close to the magnetic equat or show strong scintillations which last till early morning in a singl e patch or sometimes with a weakening or absence of scintillations for a short time duration. For the stations in the anomaly crest region o r beyond, scintillations occur in small patches with periods of no sci ntillations in between. The nocturnal variations show maximum scintill ation activity of about 50 per cent in the equatorial region which dro ps to about 30 per cent in the anomaly crest region and further reduce s to 10 per cent at Delhi, the Northern-most location in the chain. Fr om the latitudinal variations of the percentage occurrence of scintill ations, the half width of the equatorial belt of scintillations has be en found to vary with local time. It extends right up to Bombay or eve n beyond it around 2000-2200 hrs but is much narrower after midnight. The occurrence of spread-F during March-April 1991 from ionosonde data at Thumba, Waltair and Ahmedabad representing stations in the equator ial zone, intermediate zone and the anomaly crest zone have been studi ed. There is a fairly good agreement between the nocturnal variations of spread-F and scintillations.