Multifrequency studies of equatorial ionospheric scintillations at Ootacamund

Abstract

Intensity scintillations of 40 MHz, 140 MHz and 360 MHz radio beacons on board the geostationary satellite ATS-6, (at 34°E longitude), recorded at Ootacamund (magnetic dip latitude 3°N) during the ATS-6 phase II (1975-1976) are described. The scintillation index S₄, which is the normalised RMS value of the intensity fluctuations, is found to be very large at 2100-0200 h associated with equatorial spread-F, and in the forenoon and afternoon hours associated with a blanketing type of sporadic E-layer. At 40 MHz there seems to be a weak scintillation (S₄ < 0.2) present at all times of the day. Nighttime scintillations were significantly strong during the equinoctial month of October 1975. The frequency exponent, n, (with S₄∝ ƒ^-1, where ƒ is the signal frequency), was found to decrease monotonically with increasing intensity of scintillations, approaching a value of zero for saturated scintillations. The relationship between the frequency exponent and the scintillation index was found to be independent of the time of the day or the season, in spite of different kinds of irregularities being involved in the scintillation processes.