WEAK-SCATTERING THEORY FOR IONOSPHERIC SCINTILLATION

The VHF scintillations recorded at Varanasi (geomag. lat.=14 degrees 5 5'N; geomag. long.=154 degrees E) are mostly weak scintillations. Usin g a thin phase screen model for ionospheric irregularities, under the approximation of weak-scattering theory, attempts have been made to ev aluate theoretically the time evolution of the scintillation index, S- 4, for different models of electron densities. The present formulation is used to study the impact of turbulence strength, C-8, and its oute r scale length L-0 on the variation of the scintillation index S-4. Th e S-4 index increases with the increase in spectral index v but decrea ses with increasing outer scale length L-0. Diurnal, seasonal and sola r-cycle variations of the S-4 index have also been studied and it is f ound that S-4 lies between 0.2 and 0.9 as the Sun passes through solar maxima to solar minima. The results are found to be in good agreement with those reported by other workers.