The long-standing hypothesis that plasmaspheric dynamics are described by superposition of corotation and solar-wind-driven sunward convection is tested via direct comparison between plasmasphere observations and simulation output. The observations consist of global plasmasphere images produced by the IMAGE extreme ultraviolet (EUV) imager during plasmasphere erosion on 2 June 2001. The simulation is a plasmapause evolution model driven by a time-varying Volland-Stern (VS) electric potential distribution. On the dawnside and much of the nightside the model matches the EUV plasmapause position to within 0.2--0.5 earth radii (RE). Near dusk the model plasmapause is about 0.7--1.2 RE farther out than the EUV plasmapause, suggesting that an improved model should include the duskside flow enhancement known as the sub-auroral polarization stream (SAPS). We demonstrate that including a simplified ad-hoc SAPS potential can correct the model on the duskside.