Much large-scale fluid structuring in the ionosphere has been attributed to the flutelike Rayleigh-Taylor and E¿B gradient drift instabilities. The finite extent of the ionosphere and the spatial variation of plasma parameters within the ionosphere suggest that these instabilities can be expected to vary along magnetic field lines. The variations are taken into account by assuming a nonzero component of wave number parallel to the ambient magnetic field. The accompanying electric fields are not purely electrostatic but imply mode magnetic fields that may permit plasma transport across density gradients that are larger than classical cross-field diffusion. This enhanced diffusion, which is most effective for sufficiently large and tenuous plasma clouds, can limit the minimum size of striations to a larger value than classical considerations alone permit. Finite parallel wave number has the additional effect of allowing ion free energy to be transferred to parallel electron motion and so the Rayleigh-Taylor and E¿B gradient drift instabilities can contribute to structuring at conjugate points along magnetic field lines where electron energy is deposited. Also, the transfer of free energy indicates that long-term structural persistence requires a continuous source of ion free energy. Some finite parallel wavelength effects, particularly those relating to transport, can be included in present two-dimensional striation simulations.