Localized modes with zonal neutral wind, diffusion, and shear in equatorial spread F

Abstract

The effects of a zonal neutral wind, diffusion, and shear flow on perturbations in equatorial spread F (ESF) are examined. A central goal of this work is to consider effects that arise from inhomogeneity of the ionosphere along the magnetic field lines of the Earth. The perturbations discussed here have cross‐field wavelengths which are small compared with the cross‐field inhomogeneity scales of the equilibrium. Inhomogeneity in the zonal wind velocity along a magnetic field line gives rise to a vertical Pedersen current. When the effects of shear are neglected, this current can increase or decrease the growth rate of the perturbations by an amount proportional to the difference between the zonal wind speed and the equilibrium plasma drift speed divided by the ionospheric scale height. Diffusion terms proportional to the product of the ion cross‐field mobility and the electron parallel mobility are found to increase the growth rate of ESF by reducing molecular ion region loading. Effects of shear are considered using the model of Fu et al. [1986], in which the perturbations are driven to smaller scales over time. The “quasi‐local” temporal behavior of the cold kilometer scale instability is described, while retaining, in an approximate way, the effects of the cross‐field scale size dependent mapping efficiency described by Rappaport [1996]. In the diffusion regime a variational principle describing the decay of the perturbations is given. This model incorporates the effects of both parallel and cross‐field diffusions, as well as inhomogeneity along the field line.