Temporal variation of amplitude scintillations on VHF and UHF signals arising due to the presence of irregularities in the night time equatorial F region is studied during the early and late phases of evolution of these irregularities. Power spectra are calculated for the amplitude of 40, 140 and 360 MHz signals from a geostationary satellite ATS-6 measured at an equatorial station Ootacamund (11.4°N, 76.7°E, magnetic dip 6°) for the growth and decay phases of three scintillation events. The roll-off portions of the power spectra in the decay phase are found to be significantly steeper than those in the growth phase in all the cases. From a rough estimate of the irregularity drift speed obtained from the observed Fresnel frequency in each case, the steepening of the roll-off portion in the decay phase can be attributed to the dissipation of irregularities with wavelengths of several hundred meters in this phase. The slope is found to be steepest for the decay phase of that event which is associated with the highest irregularity drift speed.