Power spectra of weak phase scintillations on a 140-MHz signal, transmitted from the geostationary satellite ATS 6 and observed during premidnight and postmidnight periods at an equatorial station Ootacamund (magnetic dip 6¿N), show that the nighttime equatorial F region irregularities in the wavelength range of about hundred meters to a few kilometers exhibit a two component power law spectrum. The long- and short-wavelength spectral indices and the break scale at which the transition from a shallow to a steep slope occurs are determined self-consistently using both the phase and amplitude scintillation data. As the power spectra of phase scintillations do not exhibit the effect of Fresnel filtering, they provide fairly accurate estimates of the spectral indices and the break scale. These estimated parameters are utilized in a model calculation of the dependence of the S4 index on signal frequency based on weak scattering theory. The accuracy of the estimates is borne out by the agreement of the result of this calculation with the signal frequency dependence of S4 derived from the observed amplitude scintillations on simultaneously transmitted 140- and 360-MHz signals for all the events studied. The spectral indices and break scales determined here fall within the ranges obtained from in situ observations.