Power spectral studies of VHF ionospheric scintillations near the crest of the equatorial anomaly in India

Abstract

Scintillation of VHF (250 MHz) signals from FLEETSAT (73E) was recorded at the equatorial anomaly crest location, Rajkot, in India during 1991-93. The irregularity parameters: S4 index, and fade rate and spectral parameters: spectral slope and upper roll off frequency, fu are derived and their variation with time of night and with solar activity studied. S4 index and fade rate increase with solar activity. The spectral slope does not show any systematic dependence on solar activity but fu increases with solar activity. These results, when interpreted in terms of Fresnel size, indicate a shifting of irregularity spectrum towards longer scales in low solar activity period. The temporal variation of these parameters, after the irregularity generation phase, also indicate preponderence of large (kilometer) scale irregularities around midnight—post-midnight period, consistent with earlier simultaneous radar and scintillation observations at Jicamarca. The solar activity dependence of S4 index is interpreted as due to the variation of background F-region plasma density variations. The results suggest that when strong scattering causes strong scintillations, fu is not representative of the Fresnel scale.