Observations and Modelling of Nocturnal Ionospheric Irregularities in Low Latitude Region

The analysis of VHF scintillation recorded at Varanasi shows that the ionospheric irregularities over Varanasi are mostly of small duration <30 minutes and are frequently observed during pre-midnight periods. During magnetically disturbed days there is a marked reduction in the occurrence rate of scintillations. The increase of solar activity generally-increases the depth of scintillation. Auto-correlation functions and power spectra of scintillations predict that the mean spectral index is ∼4, and the size of the overhead ionospheric irregularities varies from 200-500 m having drift velocity between 75 m/sec to 200 nl/sec. Drift velocities and spectral indices of ionospheric irregularities derived by us agree well with the results obtained by other workers. It is shown that the thin phase screen model valid for weak scattering could explain the observed features of scintillation. The scintillation index S4, strength of turbulence Cs and its dependence on various parameters are studied and it is found that the S4 index increases with increase in spectral index and decreases with increase in outer scale length Lo.