We use results from a coupled thermosphere-ionosphere-plasmasphere model to illustrate a new mechanism for the formation of a large-scale polar cap patch of ionization arising from a rapid polar cap expansion. The patch, having an ion density at the F2 peak twice that of the background plasma, is isolated within the polar cap by a depletion due to rapid flows in the cusp region. Such depletions caused by rapid flows are generally ascribed to increases in the temperature dependent rates of plasma loss reactions due to ion frictional heating. In this paper, we demonstrate, however, that the depletion at the F peak is caused primarily by large upward (field aligned) plasma flows. These flows also give rise to an increase in the height of the F peak in the depleted region. The morphology of the patch in the F region is subsequently modulated by the return flow of plasma down previously depleted flux tubes as they convect across the polar cap, so that the enhancement decreases rapidly in both magnitude and spatial extent. Time histories for the plasma density profiles above the EISCAT Svalbard radar and the optical observatory at Eureka are presented as examples of what ground-based observers might be expected to measure under these circumstances.¿ 1997 American Geophysical Union