Adiabatic and isothermal ion-acoustic speeds of stabilized Farley-Buneman waves in the auroral E-region

The influence of ion and electron energetics on the propagation speeds of stable Parley Buneman waves which are excited by E × B drifts in the auroral E-region is studied theoretically in the fluid limit, with the effects of anomalous collisions on electron thermal conduction included for the first time. In particular, the ratio of the phase speed of waves, stabilized by enhanced diffusion effects, to the isothermal ion-acoustic speed are calculated for realistically modelled E-region ion and electron temperatures, as functions of altitude, flow velocity and wavelength. It is found that the phase speeds of these stabilized waves begin to increase above isothermal ion-acoustic speeds as wave frequencies increase to values where they are comparable with the electron inelastic collision frequency. However, at still higher frequencies their phase speeds tend to fall back towards their isothermal values due to the increasing effects, with increasing wavenumber, of electron thermal conductivity. It is also found that the phase speeds are not always isotropic with respect to flow angle. The relationship between the predictions of the present fluid theory and a previous kinetic theory calculation is also briefly discussed.