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

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 spo-radic E (Es) layer located by a sheared wind field, which experiences the same electric fields as the F layer. This con-figuration is subject to two large-scale (e.g.>10 km) “layer instabilities”: one of the F layer known as the Perkins in-stability, and another of the Es layer which has been called the Es layer instability. Electric fields on scales larger than (about) 10 km map very efficiently between the Es and F layers, and the two instabilities have a similar geometry, al-lowing them to interact with one another. As shown through a linear growth rate analysis, the two most important param-eters governing the interaction are the relative horizontal ve-locity between the Es and F layers, and the integrated con-ductivity ratio 6H /6PF, where 6H and 6PF are the field line integrated Hall conductivity of the Es layer, and the field line integrated Pedersen conductivity of the F layer, respec-tively. For both large and small relative velocities the growth rate was found to be more than double that of the Perkins instability alone, when 6H 6PF =1.8. However, the characteris-tic 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 ex-plore in this article the nonlinear evolution of the unstable coupled system subject to a 200 km wavelength initial per-turbation of the F layer, using a two-dimensional numerical solution of the two-fluid equations, as a function of relative horizontal velocity and 6