Parallel current effects on two‐stream electrojet plasma instabilities

We consider the effects of a field‐aligned electron drift on the development of two‐stream plasma instabilities which may have applications to the auroral electrojet. It is found that the inclusion of a parallel drift modifies the threshold criteria for the onset of the Farley‐Buneman instability and may cause excitation of oblique ion sound waves. However, the phase velocity of the linearly excited waves remains greater than or equal to the local ion acoustic velocity. We apply these results to an auroral E region and show that they may explain some observations of auroral irregularities, for example, the observation of irregularities which are not highly field aligned and the observation of ion acoustic wavelike irregularities for ambient electric fields below the critical value.