Ground magnetic field perturbations recorded by the CANOPUS magnetometer network in the 7 to 13 MLT sector are used to examine how reconfigurations of the dayside polar ionospheric flow take place in response to north-south changes of the IMF. During the 6-hour interval in question IMF Bz oscillates between ¿7 nT with about a 1-hour period. Corresponding variations in the ground magnetic disturbance are observed which we infer are due to changes in ionospheric flow. Cross correlation of the data obtained from two ground stations at 73.5¿ magnetic latitude, but separated by ~2 hours in MLT, shows that changes in the flow are initiated in the prenoon sector (~10 MLT) and then spread outward toward dawn and dusk with a phase speed of ~5 km s-1 over the longitude range ~8 to 12 MLT, slowing to ~2 km s-1 outside this range. Cross correlating the data from these ground stations with IMP 8 IMF Bz records produces a MLT variation in the ground response delay relative to the IMF which is compatible with these deduced phase speeds. We interpret these observations in terms of the ionospheric response to the onset, expansion, and decay of magnetic reconnection at the dayside magnetopause. ¿ American Geophysical Union 1992