The linear stability and nonlinear evolution of plasma enhancements in arbitrary ambient electric fields in the diffuse auroral F region ionosphere have been studied using analytical and numerical simulation techniques. Our results indicate that equatorward convecting plasma slabs initially limited in latitudinal extent are primarily destabilized on their poleward sides by a combination of the effects of convection and field-aligned currents. Furthermore, we find that the plasma enhancements break up into primary striationlike structures (elongated in the north-south direction for equatorward convection) which can form and cascade from large (≃100 km) to smaller (≃3 km) scale sizes on the order of an hour. The primary and associated smaller-scale structures can be oriented in either the north-south or east-west (L shell alignment) direction depending on the ambient electric field magnitude and direction. For wave numbers (kx, ky) in Fourier space corresponding to the east-west and north-south directions, respectively, the one-dimensional spatial power spectra of the irregularities in the east-west direction P(kx) ∝ kx−nx with nx ≃ 2-2.5 for 2π/kx between 100 km and 3 km while in the north-south direction, P(ky) ∝ ky−ny with ny ≃ 2 for 2πsol;ky between 256 km and 3 km.