A. Bhattacharyya

Development of intermediate scale structure near the peak of the F region within an equatorial plasma bubble

Scintillation observations are used to study the evolution of intermediate scale (~100 m–few kilometers) irregularities through growth of the Rayleigh-Taylor (R-T) instability on the bottom side of the post-sunset equatorial F region during magnetically quiet periods. Amplitude scintillations on a VHF signal from a geostationary satellite, recorded by spaced receivers at an equatorial station, are used to compute as a function of local time: (1) the coherence scale length for spatial variations of intensity in the ground scintillation pattern, which is linked with the spectrum of the intermediate scale irregularities near the peak of the equatorial F region that contribute the most to the observed scintillations; and (2) the “random velocity”, which accounts for the de-correlation of the spaced receiver signals. The relationship between the coherence scale length and the random velocity for saturated scintillations at different local times suggests that (1) the random velocity is linked with fluctuations in the drift velocity of the irregularities caused by the perturbation electric fields associated with the R-T instability rather than structural changes in the intermediate scale irregularities, (2) the spectrum of intermediate scale irregularities in the equatorial F peak region tends to be shallowest after the decay of the perturbation electric fields associated with the R-T instability, and (3) evolution of intermediate-scale irregularity spectrum in the equatorial plasma bubble near the equatorial F region peak depends on season and solar flux. These have implications for observation of low-latitude L-band scintillations.

Development of intermediate‐scale structure at different altitudes within an equatorial plasma bubble: Implications for L‐band scintillations

An important aspect of the development of intermediate scale length (~100 m – few km) irregularities in an equatorial plasma bubble (EPB) that has not been considered in the schemes to predict the occurrence pattern of L-band scintillations in low latitude regions, is how these structures develop at different heights within an EPB as it rises in the post-sunset equatorial ionosphere due to the growth of the Rayleigh-Taylor (R-T) instability. Irregularities at different heights over the dip equator map to different latitudes, and their spectrum as well as the background electron density determine the strength of L band scintillations at different latitudes. In this paper, VHF and L-band scintillations recorded at different latitudes together with theoretical modelling of the scintillations, are used to study the implications of this structuring of EPBs on the occurrence and strength of L-band scintillations at different latitudes. Theoretical modelling shows that while S4-index for scintillations on a VHF signal recorded at an equatorial station may be > 1, S4-index for scintillations on a VHF signal recorded near the crest of the equatorial ionization anomaly (EIA) generally does not exceed the value of 1 because the intermediate scale irregularity spectrum at F layer peak near the EIA crest is shallower than that found in the equatorial F layer peak. This also explains the latitudinal distribution of L-band scintillations. Thus it is concluded that there is greater structuring of an EPB on the top side of the equatorial F region than near the equatorial F layer peak.