Study of ionospheric scintillations near the crest of equatorial anomaly in India using digital data base

Abstract

The ionospheric amplitude scintillations imposed on VHF signals transmitted from geostationary satellite FLEETSAT (73°E) were recorded at Rajkot sub-ionospheric dip lat. 15.3°N) near the crest of the equatorial anomaly in India using conventional analog recording during 1987-93. Also data were recorded using digital data acquisition system during the period August 1991 to March 1993. The latter data have been subjected to autocorrelation and power spectrum analysis. Parameters such as scintillation index <i>S</i>₄, fading rate, autocorrelation interval and spectral index have been derived from each 5-min data length satisfying certain selection criteria. Scintillation index <i>S</i>₄ is maximum (>1) around the generation phase at pre-midnight hours and decrease to ~0.6 as the night progresses. The fading rate also decreases from ~120 fades/min in the pre-midnight hours to ~20 fades/min in post-midnight hours. Spectra are relatively flat (spectral index ~1) during the generation phase of irregularities and steepen (spectral index ~3 to 5) towards the 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 length. The spatial correlation length at this anomaly crest location is substantially smaller than that reported for magnetic equatorial location and comparable to that at another crest location, Ascension Island.