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The generic equilibrium configuration of the nighttime midlatitudeionosphere consists of an F layer held up against gravity by winds and/or electric fields, and asporadic E (Es) layer located by a sheared wind field, which experiences the same electricfields 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 theEs layer which has been called the Es layer instability. Electric fields on scaleslarger than (about) 10 km map very efficiently between the Es and F layers, and the twoinstabilities have a similar geometry, allowing them to interact with one another. As shownthrough a linear growth rate analysis, the two most important parameters governing the interactionare the relative horizontal velocity between the Es and F layers, and the integratedconductivity ratio ΣH/ΣPF, where ΣH and ΣPF are the fieldline integrated Hall conductivity of the Es layer, and the field line integrated Pedersenconductivity of the F layer, respectively. For both large and small relative velocities thegrowth rate was found to be more than double that of the Perkins instability alone, whenΣHΣPF=1.8. However, the characteristic eigenmode varies considerablywith relative velocity, and different nonlinear behavior is expected in these two cases. As afollow up to the linear growth rate analysis, we explore in this article the nonlinear evolutionof the unstable coupled system subject to a 200 km wavelength initial perturbation of the Flayer, using a two-dimensional numerical solution of the two-fluid equations, as a function ofrelative horizontal velocity and ΣHΣPF. We find that whenΣHΣPF⪝0.5 the Perkins instability is able to controlthe dynamics and modulate the F layer altitude in 2 to 3 h time. However, the electricfields remain small until the altitude modulation is extremely large, and even then they are notlarge enough to account for the observations of large midlatitude electric fields. WhenΣHΣPF⪞1the Es layer becomes a major contributorto the F layer dynamics. The Es layer response involves the breaking of a wave, withassociated polarization electric fields, which modulate the F layer. Larger electric fieldsform when the relative velocity between the Es and F layers is large, whereas largermodulations of the F layer altitude occur when the relative velocity is small. In the lattercase the F layer modulation grows almost twice as fast (for ΣHΣPF=1)as when no Es layer is present. In the former case the electric fields associated with theEs layer are large enough to explain the observations (~10 mV/m) , but occur overrelatively short temporal and spatial scales. In the former case also there is evidence that theF layer structure may present with a southwestward trace velocity induced by Es layermotion

annales geophysicae

Generation of mesoscale &amp;lt;I&amp;gt;F&amp;lt;/I&amp;gt; layer structure and electric fields by the combined Perkins and &amp;lt;I&amp;gt;E&amp;lt;sub&amp;gt;s&amp;lt;/sub&amp;gt;&amp;lt;/I&amp;gt; layer instabilities, in simulations