Statistical features of overhead ionospheric irregularities and its generation mechanism at low latitude

VHF scintillations recorded during the period January 1991–December 1993 in the declining phase of a solar cycle and April 1998–December 1999 in the ascending phase of the next solar cycle at low latitude station Varanasi (geom. lat.=14°55′N, long.=153°55′E) have been analyzed to determine the statistical features of overhead ionospheric irregularities. Derived spectral index ranges between −2 and −8 and characteristic length of these irregularities varies from 100 to 1800 m, having drift velocity between 75 and 200 m/s. In some cases, spectra displaying a double slope have been observed, which indicate an inner and an outer region for the power-law irregularities. We have computed growth rate using the parameters appropriate to the ionosphere overhead of Varanasi for the Rayleigh–Taylor instability in the intermediate scale size range. It is noted that the irregularity could develop to its full size in the altitude range 250–350 km and under certain conditions may explain the observed features of the irregularities.