The linear stability and eventual nonlinear saturation of plasma depletions in the high-altitude, F layer ionosphere are modeled via two-fluid equations for a plasma consisting of electrons and a single species of ions. Major emphasis is given to the nonlocal properties of the perturbations, permitting the description of relatively long wavelength modes (k⊥L→1, where L is a characteristic plasma scale length and k⊥ is the wavevector component perpendicular to the magnetic field), a regime not accessible under the often used ''local approximation.'' The condition for onset (marginal stability), 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 illustrate the dynamics of the depletions, and estimates are given for the saturated amplitudes. ¿ American Geophysical Union 1996