The mutual coherence function Γ2, or the second moment of the complex amplitude of a radio wave which traverses through equatorial F region irregularities, is computed from amplitude and phase scintillation data. Theoretically, the equation satisfied by the coherence function has an analytic solution over the whole range of scintillation strength. This solution is directlly related to the structure function for the phase fluctuations produced by the irregularities. Hence the shape of the correlation function for variations in the total electron content along the signal path can be derived from the computed values of Γ2. With a suitable power-law model for the irregularities, an “intermediate break scale,” 10, as well as the rms density fluctuation are deduced from a comparison of computed values for short time lags with those expected from theory. During a postsunset scintillation event, 10 is found to increase with local time. In the context of the generalized Rayleigh-Taylor instability, which is the likely source of the irregularities, this increase may be attributed to a decline in the effective electric field prevailing in the region of the irregularities.
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