Simulations of Secondary Farley-Buneman Instability Driven by a Kilometer-Scale Primary Wave: Anomalous Transport and Formation of Flat-Topped Electric Fields

Abstract

Since the 1950s, high frequency and very high frequency radars near the magnetic equator have frequently detected strong echoes caused ultimately by the Farley-Buneman instability (FBI) and the gradient drift instability (GDI). In the 1980s, coordinated rocket and radar campaigns made the astonishing observation of flat-topped electric fields coincident with both meter-scale irregularities and the passage of kilometer-scale waves. The GDI in the daytime E region produces kilometer-scale primary waves with polarization electric fields large enough to drive meter-scale secondary FBI waves. The meter-scale waves propagate nearly vertically along the large-scale troughs and crests and act as VHF tracers for the large-scale dynamics. This work presents a set of hybrid numerical simulations of secondary FBIs, driven by a primary kilometer-scale GDI-like wave. Meter-scale density irregularities develop in the crest and trough of the kilometer-scale wave, where the total electric field exceeds the FBI threshold, and propagate at an angle near the direction of total Hall drift determined by the combined electric fields. The meter-scale irregularities transport plasma across the magnetic field, producing flat-topped electric fields similar to those observed in rocket data and reducing the large-scale wave electric field to just above the FBI threshold value. The self-consistent reduction in driving electric field helps explain why echoes from the FBI propagate near the plasma acoustic speed.