This paper presents a quantitative explanation of storm-enhanced density (SED). The plasma's root origin lies in the fully sunlit equatorial ionosphere where a penetrating zonal electric field drives plasma upward so fast that it cannot recombine. This plasma spills over into the anomaly and then is driven poleward by a penetrating zonal electric field. However, the poleward field cannot extend into the dayside due to high conductivity and the flow stagnates, causing plasma to build up in a narrow channel along the dusk terminator and flow into the convection pattern. It is remarkable that plasma finds its way into the polar cap from the daytime equator. We believe that this plasma structure is connected to the plasmaspheric tails reported in the literature.