Diffuse radar aurora and the gradient drift instability

The plasma fluid equations used by Sudan and his co-workers to explain the small-scale secondary irregularities observed in the equatorial electrojet are applied to the diffuse radar aurora. It is shown that the increased magnitude of the ambient magnetic field in the auroral zone and the decreased ion-neutral collision frequency, due to the higher altitude at which diffuse radar aurora occurs, combine to stabilize secondary two-stream irregularities over the expected range of electron drift velocities and primary irregularity amplitudes. It is also shown that secondary gradient drift irregularities are unstable for electron drift velocities in the vicinity of the ion acoustic velocity and that in addition to primary two-stream irregularities, primary gradient drift irregularities may be generated in the auroral E layer density gradients. These results are used to explain some of the spectral observations of the diffuse radar aurora.