Abstract:
The ambient conditions required for the onset of equatorial spread F (ESF) irregularities have been the focus of
numerous studies. However, for prediction of the strength and duration of scintillations on VHF and higher frequency radio
signals, it is necessary to know how ambient conditions influence the growth and decay of intermediate scale (~ 100 m - 1
km) irregularities in the post-sunset equatorial ionosphere. For this purpose, the coherence scale length of the ground
scintillation pattern of intensity is computed from spaced receiver measurements of intensity scintillations on a VHF signal
transmitted from a geo-stationary satellite and recorded at an equatorial station. For weak scintillations (S4 ≤ 0.5), the
coherence scale length is determined by the Fresnel scale length and the irregularity spectrum. The observed scintillations are
an integrated effect of all the irregularities in the path of the signal, with maximum contribution coming from the region of
the F layer peak. Hence for weak scintillations, the coherence scale length depends on the height of the equatorial F layer
peak, which changes with time. For saturated scintillations (S4 ≥ 1), the coherence scale length becomes independent of the
height of the scintillation-producing irregularities, and is determined by the strength of the irregularities and the spectral slope
of a power-law irregularity spectrum. Dependence of the computed coherence scale length for weak as well as saturated
scintillations, on season and solar flux, is interpreted in the context of theoretical results derived from modeling of
scintillations. It is found that for days with 10.7 cm solar flux greater than 150, the shallowest irregularity spectrum near the
equatorial F layer peak is likely to be found after midnight, whereas for days with lower solar flux, the irregularity spectrum
becomes shallowest before midnight.
