Abstract
Abstract. The data from ground based experiments conducted during the 2005 SpreadFEx campaign in Brazil are used, with the help of theoretical model calculations, to investigate the precursor conditions, and especially, the role of gravity waves, in the instability initiation leading to equatorial spread F development. Data from a digisonde and a 30 MHz coherent back-scatter radar operated at an equatorial site, Sao Luis (dip angle: 2.7°) and from a digisonde operated at another equatorial site (dip angle: −11.5°) are analyzed during selected days representative of differing precursor conditions of the evening prereversal vertical drift, F layer bottom-side density gradients and density perturbations due to gravity waves. It is found that radar irregularity plumes indicative of topside bubbles, can be generated for precursor vertical drift velocities exceeding 30 m/s even when the precursor GW induced density oscillations are marginally detectable by the digisonde. For drift velocities ≤20 m/s the presence of precursor gravity waves of detectable intensity is found to be a necessary condition for spread F instability initiation. Theoretical model calculations show that the zonal polarization electric field in an instability development, even as judged from its linear growth phase, can be significantly enhanced under the action of perturbation winds from gravity waves. Comparison of the observational results with the theoretical model calculations provides evidence for gravity wave seeding of equatorial spread F.
Highlights
Equatorial spread F post-dusk development occurs under electrodynamical processes unique to the sunset transition in the E- and F layers of the ionosphere
The analysis focused on the ambient conditions of the evening IT, in terms of the prereversal enhancement in zonal electric field/vertical drift, the F layer bottom-side density gradients, and the density fluctuations in the form of gravity wave (GW) that were present, as precursors to post sunset spread F development
The main conclusions from this study are the following: 1. The features of GWs as observed in their signatures in the F layer bottom-side densities are in general agreement with those predicted by theory for GWs propagating from tropospheric convective sources as identified from complementary experiments conducted during the SpreadFEx campaign; 2
Summary
Equatorial spread F post-dusk development occurs under electrodynamical processes unique to the sunset transition in the E- and F layers of the ionosphere. The Rayleigh-Taylor interchange instability (RTI), or the generalized gradient drift instability (GDI), is believed to be the mechanism by which an instability initiated at the F layer bottomside develops into flux tube aligned plasma depletions (with their cascading irregularity structures) rising to the topside of the F region. The signatures of these plasma irregularities have wide ranging features as registered by radars, digisondes, optical imagers, GPS receivers and space-borne detectors. ESF presents a large degree of variability at medium term (seasonal), long term (solar cycle) and short term (day-to-day) scales The last of these is the least understood aspect and constitutes the most challenging problem for present day ESF investigations.
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