Narrow Width Farley‐Buneman Spectra Above 100 km Altitude

Abstract

AbstractFor spectra associated with full turbulence the observed mean phase velocity of unstable Farley‐Buneman waves has been found not to exceed the ion‐acoustic speed, cs. This has been attributed to various nonlinear processes. However, weakly turbulent modes are also excited on the edge of the “instability cone.” These modes have to be actual eigenmodes predicted by linear instability theory near‐threshold conditions. Unlike the modes that are associated with strong turbulence, these weakly turbulent modes are affected by the ion drift. This can make the Doppler shift of narrow spectra reach as high as the E × B drift velocity in the upper portion of the unstable layer at small aspect angles. Slow narrow spectra are also predicted nearer the E direction. We have produced a model of the Doppler shift of narrow‐width spectra under various electric field conditions above 100 km altitude. While the fluid dispersion relation is used to clarity the physics, we have also found the eigenmodes from an accepted kinetic dispersion relation. The calculations include a new model of the ion‐acoustic speed based on an empirical model of the electron temperature and of ion frictional heating under strong electric field conditions. The model provides an explanation for various VHF observations of the Doppler shift of narrow spectra that have been called “Type III spectra” and “Type IV spectra” in the existing literature.