The origins, transport, and decay of large-scale (>10 km) F region density irregularities were theoretically studied using a high-latitude time dependent ionospheric model. Such density irregularities (blobs) have been found both in the polar cap and the auroral zone. The model study, which focuses on blobs being produced by auroral precipitation, shows that the observed energy fluxes can readily account for the blob densities if a plasma flux tube is exposed to the precipitation for 5-10 min. Once the flux tube is tranported away from the source, the F region density profile recovers its shape on a time scale of 10-20 min. Hence after this time period it is not possible to determine whether hard or soft precipitation was the source of the blob. Once created a blob will maintain its relative size for many hours, since it decays at the same rate as the background plasma. Blobs are removed by being transported into a region of high production, where the new background density exceeds the old blob density. The high hm F2's observed with some blobs need not necessarily imply soft precipitation; induced upward plasma drifts from neural winds or E¿B convection can readily account for them. Since the high latitude ionosphere contains discrete auroral forms in the oval and polar cap and since plasma is continually convecting through these regions, blobs are the rule rather than the exception.