Interaction of plasma drifts: A hypothesis for equatorial spread F occurrence

Studies on the generation mechanisms of post-sunset ionospheric irregularities, which constitute a threat to trans-ionospheric radio propagation, particularly at the African sector, are quite sparse. Recently, a GNSS scintillation monitor was co-located with an existing HF Doppler receiver at the University of Lagos (geographic: 3.27° E, 6.48° N; dip latitude −1.72°), Nigeria. The availability of data from both instruments for the period March–September 2022 provided an opportunity to study ionospheric irregularities using the datasets from the instruments. The post-sunset rise of the ionosphere was obtained from the HF Doppler monitor, while plasma bubble depth was determined from GPS total electron content (TEC) measurements. Ionospheric scintillation, determined from amplitude scintillation index (S4 index), was observed to peak in amplitude at latitudes 7°–10° south of the dip equator with a lower peak occurring in the vicinity of the dip equator. Amplified wave structures were consistently identified in filtered time series of slant TEC from GPS satellites in view of the receiver during the time of scintillation occurrence. These wave structures were predominant at about the time of local sunset, when shear flow between eastward and westward plasma drifts is known to be greatest. Rising plumes were also observed during local sunset, and the results suggest that these plumes could be signatures of interacting eastward and westward plasma drifts. Linear correlation analysis of S4 index and wave amplitude yielded a higher correlation index, compared with a similar analysis of S4 index and post-sunset rise. It is postulated that even when the post-sunset rise is low, the interaction of waves can generate rising plumes, which in turn facilitate the development of the Rayleigh-Taylor instability and ionospheric irregularities.