Signal frequency dependence of ionospheric amplitude scintillations

The S4 index, which is a measure of the strength of amplitude scintillations, is assumed to vary with signal frequency f according to S4 ∝ f−n, and the frequency exponent n is derived from observed equatorial amplitude scintillations on 40‐ and 140‐MHz signals. A large data base has been used to study the variation of this exponent n(40/140) with the S4 index on the 140‐MHz signal for both daytime and nighttime scintillation events. Although the equatorial E and F region irregularities, which cause the observed scintillation events, have different characteristics, the variation of n(40/140) with S4(140) is found to be almost universal. Theoretical computation of n(40/140) for different values of the irregularity parameters yields results which are in good agreement with these observations.