We report IMAGE EUV observations of a striking undulatory motion of the plasmapause that occurred on 17 April 2002, in which a large ripple propagated westward across the duskside plasmapause. From the plasmapause motion we infer a peak E-field of ≈3 mV/m associated with the undulation, and estimate the undulation's equatorial azimuthal speed to be 0.9 rad/hr ≈ 4 RE/hr, corresponding to ≈1 km/s in the ionosphere (using an R3/2 dependence to map from the equator). Because DMSP ion drift data at 850 km altitude show the presence of a subauroral polarization stream (SAPS) flow channel ≈1 km/s at the same location and time as the undulation, we assert that SAPS was responsible for removal of plasma during the undulation. The SAPS apparently formed in response to a 19:00 UT substorm onset.