Narrow Width Farley‐Buneman Spectra Above 100 km Altitude

For 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. Plain Language Summary HF and UHF radars routinely detect turbulence in the aurora between 100 and 120 km altitudes. The turbulent structures are excited whenever the electric field produces much larger electron than ion drifts. At their largest amplitudes, the structures end up moving at the ion acoustic speed, cs, which can be much less than the electron E × B drift at times. The most turbulent modes are excited at right angles to the ambient ion drift and are not affected by it. By contrast, we deal here with weakly turbulent modes excited at viewing angles such that the ion drift can contribute to the phase velocity while the frequency obeys linear instability theory. In the upper part of the unstable E region, this introduces narrow width spectra that can move as fast as the electron E × B drift, a velocity which can be much faster than cs under strong electric field conditions. This means that narrow spectra can exhibit much faster Doppler shifts than their slower fully turbulent counterpart. We also find that narrow spectra can exhibit Doppler shifts t