Vortex streets driven by sheared flow and applications to black aurora

Black aurora often exhibits the signature of vortex structures. In this paper, it is proposed that pseudo-three-dimensional drift-like electrostatic modes can play a very important role in those phenomena. Employing a two-fluid model, we derive a set of nonlinear equations governing the dynamics of long wavelength inertial convective cells and coupled drift-acoustic waves in the presence of geomagnetic field-aligned plasma flows. It is shown that free energy stored in the latter can cause purely growing and oscillatory instabilities in the auroral ionosphere. The threshold conditions as well as expressions for the linear growth rates are presented. Furthermore, it is found that possible stationary solutions of the nonlinear equations can be represented in the form of vortex structures. It is, therefore, likely that the nonlinear low-frequency electrostatic modes, as described here, may possibly account for the coherent vortex structures within black aurora, as they are often observed during the late recovery phase of a substorm.