Simulation of the nonlinear evolution of the sporadic-E layer instability in the nighttime midlatitude ionosphere

Recently, it has been shown that the configuration of a midlatitude sporadic-E (E-s) layer at a zonal wind-shear node is unstable at night. The instability is the result of a wind-shear driven polarization process that occurs when a plane-wave perturbation in altitude is imposed on the E-s layer. The growth rate of the instability depends on the azimuthal alignment of the plane wave distortion, a feature that is reminiscent of the Perkins instability. The plane wave nature of the growing modes, combined with the azimuthal dependance of the growth rate, suggests that the instability may provide an explanation for frontal structures observed in E-s layers, which are found to consistently adopt the same azimuthal alignment preferred by the instability. In this paper the nonlinear evolution of the instability is simulated using a flux-corrected transport finite difference method for the orientation of maximum linear growth rate. It is found that the instability generates significant polarization electric fields that structure the layer; yet the structuring saturates without destroying the layer completely. Decreasing the wind shear decreases the polarization electric field and increases the evolution time scale but otherwise does not profoundly affect the structures that form in the layer. Increasing the F layer conductivity, which is assumed to map perfectly to the E-s layer, damps the instability. The results suggest the instability as a possible source of the so-called quasi-periodic (QP) echoes, which are coherent radar echoes found in the nighttime midlatitude ionosphere.