On 10 July 2000, the IMAGE EUV imager observed erosion of the nightside plasmasphere that occurred in two bursts during 5--8 UT. The plasmapause radial velocity Vpp at 2.4 MLT was extracted from the time sequence of EUV images. We show that intervals of Vpp < 0 (i.e., erosion) are correlated with intervals of southward (Swd) interplanetary magnetic field (IMF), if the solar wind and IMF data are time-delayed by 30 minutes (in addition to a 3.7-minute delay for propagation to the magnetopause). This suggests that coupling between the solar wind and the plasmapause, involving processes in the ionosphere and magnetotail, takes about 30 minutes. A 6:40 UT magnetosphere compression may have hurried the onset of the second erosion.