Generation of mesoscale F layer structure and electric fields by the combined Perkins and E_s layer instabilities, in simulations

Abstract. The generic equilibrium configuration of the nighttime midlatitude ionosphere consists of an F layer held up against gravity by winds and/or electric fields, and a sporadic E (E_s) layer located by a sheared wind field, which experiences the same electric fields as the F layer. This configuration is subject to two large-scale (e.g. >10 km) "layer instabilities": one of the F layer known as the Perkins instability, and another of the E_s layer which has been called the E_s layer instability. Electric fields on scales larger than (about) 10 km map very efficiently between the E_s and F layers, and the two instabilities have a similar geometry, allowing them to interact with one another. As shown through a linear growth rate analysis, the two most important parameters governing the interaction are the relative horizontal velocity between the E_s and F layers, and the integrated conductivity ratio Σ_H/Σ_PF, where Σ_H and Σ_PF are the field line integrated Hall conductivity of the E_s layer, and the field line integrated Pedersen conductivity of the F layer, respectively. For both large and small relative velocities the growth rate was found to be more than double that of the Perkins instability alone, when Σ_HΣ_PF=1.8. However, the characteristic eigenmode varies considerably with relative velocity, and different nonlinear behavior is expected in these two cases. As a follow up to the linear growth rate analysis, we explore in this article the nonlinear evolution of the unstable coupled system subject to a 200 km wavelength initial perturbation of the F layer, using a two-dimensional numerical solution of the two-fluid equations, as a function of relative horizontal velocity and Σ_HΣ_PF. We find that when Σ_HΣ_PF⪝0.5 the Perkins instability is able to control the dynamics and modulate the F layer altitude in 2 to 3 h time. However, the electric fields remain small until the altitude modulation is extremely large, and even then they are not large enough to account for the observations of large midlatitude electric fields. When Σ_HΣ_PF⪞1 the E_s layer becomes a major contributor to the F layer dynamics. The E_s layer response involves the breaking of a wave, with associated polarization electric fields, which modulate the F layer. Larger electric fields form when the relative velocity between the E_s and F layers is large, whereas larger modulations of the F layer altitude occur when the relative velocity is small. In the latter case the F layer modulation grows almost twice as fast (for Σ_HΣ_PF=1) as when no E_s layer is present. In the former case the electric fields associated with the E_s layer are large enough to explain the observations (~10 mV/m) , but occur over relatively short temporal and spatial scales. In the former case also there is evidence that the F layer structure may present with a southwestward trace velocity induced by E_s layer motion.