Multifrequency amplitude scintillation data at L and C bands from the equatorial region at Ascension Island are used to study the spectral index n for the scintillation index S4. The average frequency dependence of the scintillation index (S4) is found to be f−1.68. It is shown that the spectral index tends to be larger for events which occur after 2030 local time than for events which occur before 2030. This may indicate temporal evolution of the irregularity spectrum. Model studies based on a two component power law irregularity spectrum are applied to relate changes in n to spectral evolution and to show that the observations are consistent with recent in situ measurements. Possible geophysical implications are also discussed.