VHF SCINTILLATION AND SPREAD-F IN THE ANOMALY CREST REGION

Regular VHF scintillations observations over Ahmedabad, situated near the anomaly crest, have been made since March 1991 using the 244 MHz b eacon onboard the geostationary satellite Fleetsat (73-degrees-E). The data recorded during the period March 1991 to January 1992 have been analysed to study the occurrence characteristics of scintillations dur ing different seasons. Generally scintillations start around 20-21 hrs IST and occur in patches of short durations. The occurrence of scinti llation during the period of study shows equinoctial maxima with a max imum hourly occurrence value of about 35 per cent which drops to 15 pe r cent during solstices. The peak in the nocturnal variations of the s cintillation occurs around 21 hrs IST during equinoxes, around 23 hrs IST during winter and around 03 hrs IST during summer. There is a mark ed reduction in the occurrence frequency of scintillations on magnetic ally disturbed days during pre-midnight period. The occurrence of scin tillations are also compared with the occurrence of spread-F over Ahme dabad as observed by the ionosonde. Good agreement is noted between th e occurrence of spread-F and scintillation during equinoxes and winter months, however, there are significant differences during summer mont hs. Power spectra of scintillations are also studied for about 100 sam ples. Spectral index values vary from 1 to 5 with a mean value of 2.8 and show dependence on the scintillation index S4.