Study of ionospheric electron content (IEC) depletions and their association with VHF scintillations

Significant depletions in IEC data observed in the Indian sector from the Faraday rotation of VHF signals of the ETS-II and SIRIO satellites for high (1978-81) and low (1983-84) solar activity periods at a low latitude station, Waltair (17.7degN, 83.3degE), are studied. Their diurnal, seasonal and solar cycle variations in the occurrence characteristics are presented. The diurnal variations in the occurrence of depletions shows a maximum around pre-midnight hours of 2100 hrs LT. With the available data, excluding the months of satellite eclipses, the month-to-month variation in the occurrence of depletions shows maximum in February during low solar activity period and in August during high solar activity period. The amplitudes of depletions vary from 1 to 9 IEC units (1 IEC unit = 1016 el./m2) and the duration of most depletions vary from 5 to 15 min during the high and low solar activity periods. Further, the amplitudes of depletions vary linearly with their durations indicating the bubbles with long durations have large depletions. All these depletion events are invariably accompanied by amplitude scintillations. (author). 30 refs., 9 figs