NONLOCAL DYNAMICS OF THE COLLISIONAL RAYLEIGH-TAYLOR INSTABILITY - APPLICATIONS TO THE EQUATORIAL SPREAD-F

The linear stability and eventual nonlinear saturation of plasma deple tions in the high-altitude, F layer ionosphere are modeled via two-flu id equations for a plasma consisting of electrons and a single species of ions. Major emphasis is given to the nonlocal properties of the pe rturbations, permitting the description of relatively long wavelength modes (k(perpendicular to) L --> 1, where L is a characteristic plasma scale length and k(perpendicular to) is the wavevector component perp endicular to the magnetic field), a regime not accessible under the of ten used ''local approximation.'' The condition for onset (marginal st ability), as well as the spatial structure of strongly growing linear eigenmodes, are derived analytically for an ionosphere that includes a height-dependent collisional diffusion coefficient. A nonlinear model , comprising both quasilinear and three-wave coupling, is used to illu strate the dynamics of the depletions, and estimates are given for the saturated amplitudes.