Associated with the large-scale enhancement of the ionospheric convection electric field during disturbed geomagnetic conditions, solar-produced F region plasma is transported to and through the noontime cleft from a source region at middle and low latitudes in the afternoon sector. As a result of the offset between the geomagnetic and geographic poles, the afternoon sector region of strong sunward convection is shifted to increasingly lower geographic latitude throughout the interval between 12 UT and 24 UT. A snowplow effect occurs in which the convection cell continually encounters fresh corotating ionospheric plasma along its equatorward edge, producing a latitudinally narrow region of storm-enhanced plasma density (SED) and increased total electron content which is advected toward higher latitudes in the noon sector. The Millstone Hill incoherent scatter radar regularly observes SED as a spatially continuous, large-scale feature spanning local times between noon and midnight and at latitudes between the polar cap and its mid- or low-latitude source region. For local times away from noon, the latitude of most probable SED occurrence moves equatorward by 6¿ for an increase of 2 in the Kp index. During strong disturbances the topside SED is observed to be convecting sunward at ~750 m s-1 with a flux of 1014 m-2 s-1. This feature accounts for the pronounced enhancement of ionospheric density near dusk at middle latitudes observed during the early stages of magnetic stroms (called the dusk effect) and constitutes a source for the enhanced F region plasma observed in the polar cap during disturbed conditions. ¿ American Geophysical Union 1993