VORTEX STREETS DRIVEN BY SHEARED FLOW AND APPLICATIONS TO BLACK AURORA

Black aurora often exhibits the signature of vortex structures. In thi s paper, it is proposed that pseudo-three-dimensional drift-like elect rostatic modes can play a very important role in those phenomena. Empl oying a two-fluid model, we derive a set of nonlinear equations govern ing the dynamics of long wavelength inertial convective cells and coup led drift-acoustic waves in the presence of geomagnetic field-aligned plasma flows. It is shown that free energy stored in the latter can ca use purely growing and oscillatory instabilities in the auroral ionosp here. The threshold conditions as well as expressions for the linear g rowth rates are presented. Furthermore, it is found that possible stat ionary 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 ac count for the coherent vortex structures within black aurora, as they are often observed during the late recovery phase of a substorm.