STUDY OF IONOSPHERIC SCINTILLATIONS NEAR THE CREST OF EQUATORIAL ANOMALY IN INDIA USING DIGITAL DATA-BASE

The ionospheric amplitude scintillations imposed on VHF signals transm itted from geostationary satellite FLEETSAT (73 degrees E) were record ed at Rajkot (sub-ionospheric dip lat. 15.3 degrees N) near the crest of the equatorial anomaly in India using conventional analog recording during 1987-93. Also data were recorded using digital data acquisitio n system during the period August 1991 to March 1993. The latter data have been subjected to autocorrelation and power spectrum analysis. Pa rameters such as scintillation index S-4, fading rate, autocorrelation interval and spectral index have been derived from each 5-min data le ngth satisfying certain selection criteria. Scintillation index S-4 is maximum (> 1) around the generation phase at pre-midnight hours and d ecreases to similar to 0.6 as the night progresses. The fading rate al so decreases from similar to 120 fades/min in the pre-midnight hours t o similar to 20 fades/min in post-midnight hours. Spectra are relative ly flat (spectral index similar to 1) during the generation phase of i rregularities and steepen (spectral index similar to 3 to 5) towards t he decay phase. Observations using the spaced receiver technique have enabled the determination of E-W velocity of the irregularities which together with the autocorrelograms yield the spatial correlation lengt h. The spatial correlation length at this anomaly crest location is su bstantially smaller than that reported for magnetic equatorial locatio n and comparable to that at another crest location, Ascension Island.