Simulations of the dynamics and structure associated with the black aurora have been performed. We find that the nonlinear evolution of the stratified Kelvin-Helmholtz shear instability in the presence of a density gradient leads to structured vortices and electric fields in addition to large density depletions. Timescales for vortex generation, roll-up, and breaking are of the order of a second with spatial scales of the order of several hundred meters to several kilometers. Good agreement is found between simulation results and recent experimental observations.