The proposed effort will focus on extension and application of an existing, two-dimensional, numerical model of the mid- and low-latitide current systems within the ionosphere. This variable-grid model is based on magnetic field-line integrated quantities and solves for scales from global down to medium (5km) electric fields. First, the model will be used to investigate the evening perturbation to the Sq (solar quiet) current system which is responsible for the rapid rise of the ionosphere after local solar sunset. This will provide a self-consistent numerical test of the theory presented by Haerendel et al. ?1990! on the post-sunset rise. Once the numerical study of the global scale electric field is completed, a finer grid will be employed to investigate the explosive development of irregularities within the evening F Region plasma called Equatorial Spread F (ESF). While the driving mechanism of ESF is well understood (gravitational Rayleigh-Taylor instability), the initiation or "seeding of the instability process is still not established. Initially, seeding will be omitted to see if Equatorial Spread F develops from the nonlinear first order electric field variation. Then seeding from neutral- wave/ionosphere interactions such as spatial resonance effects will be examined.
