An electron thermal diffusion instability and type-3 echoes in the auroral E-region plasma : Progress in understanding ionospheric irregularities

The observational evidence on type-3 radar auroral irregularities, which are characterized by a Doppler spectrum signature with a narrow peak below ion acoustic speeds, indicates that this coherent scatter is generated under uncommon auroral plasma conditions. In an effort to understand the physics of type-3 radar aurora, we introduce a plasma instability process based on linearized fluid theory and operating during strong electron gas heating events. In these conditions, in addition to the standard terms in the momentum and continuity equations, the model includes a thermal force term proportional to B×⊇T e in the electron momentum equation, and also the energy balance equation for the electrons. This leads to a dispersion relation for low-frequency, meter-scale, plasma waves that require for their excitation threshold velocities well below the ion acoustic speed. In general, this is not a totally new instability for the E-region plasma but is relies on a new destabilizing mechanism; that is, strong electron thermal diffusion. Under the widely used assumption that the waves generated propagate at threshold velocities, numerical results show the phase velocities of the waves to be relatively invariant inside the unstable laye of the heated region which, in turn, leads to a narrow spectrum at sub-ion acoustic Doppler velocities. In this paper, we discuss the theory as well as the physical nature of the destabilizing process, and obtain theoretical predictions at instability threshold conditions. It is shown that the electron thermal diffusion instability mechanisms can account for several of the type-3 radar auroral properties