Analysis of a long period fading component in saturated VHF amplitude scintillation observed at Ascension Island

Previous studies of saturated VHF amplitude scintillations on geostationary satellite beacon signals measured at Ascension Island have shown that the temporal coherence time of the saturated scintillation is generally on the order of several tenths of a second in the post-sunset to pre-midnight period. These scintillation events are often associated with equatorial plasma bubbles. In this paper we show that this rapid fading is accompanied by slow fluctuations on time scales on the order of several minutes having peak to peak amplitudes of ~ 6 dB. These fluctuations are associated with the scale sizes in the diffraction pattern on the ground that are more than 10 times the Fresnel dimension. Analysis of this fading component shows that a characteristic long period fading signature is often present at the very beginning and end of a patch. By modeling this signature, it is shown to be consistent with what is expected for a wave encountering the sharp, vertically elongated edge of a large scale depletion in electron density. In addition, a composite fading spectrum covering 4 1 2 decades in frequency is shown to agree closely with strong scintillation model spectra computed using a two component power-law spectrum model for the ionospheric irregularities. This composite spectrum extends previously reported spectra down in frequency by 2 decades, and offers the rare opportunity to verify predictions based on scintillation theory.