The rate of cross-field plasma diffusion in the F region ionosphere is significantly increased when the magnetic field lines thread a highly conducting E region below. This reduces the lifetime of small-scale F region electron density irregularities in the polar ionosphere where the presence of a highly conducting E region is comonplace. A simple mmodel is developed to describe the effects of a conducting E layer on classical F region plasma diffusion. In the absence of an E region, the difference in ion and electron diffusion rates leads to a charge separation and, hence, to an electrostatic field that retards ion diffusion. When the highly conducting magnetic field lines are tied to a conducting E region, however, electrons can flow along B to reduce the ambipolar diffusion electric field, and ions can proceed perpendicular to B at a rate approaching their own (higher) diffusion velocity. It is shown that the enhanced total diffusion rate that results depends strongly on the height of the F layer and on the ratio of the E to F region Pedersen conductivities. Although the enhanced classical diffusion rate hastens the removal of irregularities once their production source is removed, it is not a strong enough damping mechanism to prevent instabilities from operating routinely in the polar ionosphere. However, the E region probably plays in important role in determining the scale size of the irregularities that are favored. E region 'images' may be important for low E region electron densities and small scale sizes, in which case the diffusion rate is lowered. However, if the E region conductivity is high, the presence of images only reduces the F region cross-field plasma diffusion rate by about 25% from the ion rate. We hypothesize that the spectrum of high-latitude plasma density irregularities is controlled at large scale (&lgr;>10 km) by structured soft electron precipitation and classical diffusion. Smaller scale waves are produced by plasma instabilities operating on the edges of the large scale structures. The generalized ?¿? instability (including the current convective process) acts to strengthen waves in the intermediate scale size (100 m?&lgr;?10 km) in regions where the geometry is appropriate or where field-aligned currents are significant. Universal drift waves transfer energy from the intermediate scale to smaller structures but are ineffectual at large scales. The classical diffusion process described herein is applied (in conjunction with a model of irregularity production and convection) to the problem of explaining the morphology of the large scale high-latitude irregularities in a companion paper (Kelley et al., this issue). The anomalous diffusion due to the instabilities mentioned above is also described in more detail.