[1] The configuration of a midlatitude sporadic-E (E s ) layer at a zonal wind-shear node has been assumed to be a stable equilibrium, as long as the wind shear is hydrodynamically stable. In this paper, we show that this equilibrium is in fact unstable, at night, to electrodynamical forces that arise when plane wave perturbations in altitude or density are imposed on the E s layer. We show that the growth rate of the instability depends on the azimuthal alignment of the phase fronts of the plane-wave distortion, a feature that is reminiscient of the Perkins instability [Perkins, 1973]. The dependence on azimuth is consistent with observations of structure in E s layers that show the same preferred alignment.INDEX TERMS: 2471 Ionosphere: Plasma waves and instabilities. Citation: Cosgrove, R. B., and R. T. Tsunoda, A direction-dependent instability of sporadic-E layers in the nighttime midlatitude ionosphere, Geophys. Res.

[1] Tsunoda and Cosgrove [2001] recently pointed out that the F layer and sporadic E (E s ) layers in the nighttime midlatitude ionosphere must be considered electrodynamically as a coupled system in light of the presence of a Hall polarization process in E s layers [Haldoupis et al., 1996;Tsunoda, 1998; Tsunoda, 2001, 2002a] and the fact that kilometer-scale electric fields map efficiently between the E and F regions. They further noted the apparent presence of positive feedback between processes in those regions. Tsunoda [2002b, 2003] have since shown that E s layers are unstable with properties not unlike those of the Perkins instability in the F region [Perkins, 1973], thus providing yet another reason to consider the behavior of the coupled system. In this paper we show mathematically that the coupled system is indeed unstable with properties that are not only related to but distinct from those of the individual instabilities. A key finding is that this coupled response significantly enhances the growth rate of F region structures, beyond that of the Perkins instability.

Abstract. We describe a new, self-consistent scenario in which sporadic-E doublets, height bands of upward-displaced F-layer profiles, F-region plasma depletions, and radar backscatter plumes, are all manifestations of a coupled electrodynamical response by the nighttime midlatitude ionosphere to the presence of a traveling ionospheric disturbance (TID). We show that the response consists of (1) formation of image plasma structure in the E region, (2) initiation of a Hall-current-driven polarization process by the E-region plasma structure, and (3) mapping of the polarization electric field to the F region, where it strengthens the electrical properties of the TID that initiated the E-region processes. This scenario provides ready answers for several, hitherto puzzling, questions and a basis for new directions on this research topic.

[1] 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 windshear 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. Citation: Cosgrove, R. B., and R. T. Tsunoda, Simulation of the nonlinear evolution of the sporadic-E layer instability in the nighttime midlatitude ionosphere,