Changes in ionospheric flow patterns provide direct observations of how the magnetosphere-ionosphere system is coupled to the IMF. The nature and location of reconnection on the magnetopause is communicated to the ionosphere, which responds by reconfiguring global convection patterns. We investigate the dynamics of the changing global ionospheric convection patterns using a large two-dimensional array of ground magnetometers. We have found that for sharp north to south transitions of the IMF, dayside ionospheric convection patterns globally reconfigure themselves in timescales as short as 8 minutes. We find that all local times initially respond within 1--2 minutes of the first detectable response in the ionosphere. The length of time (after the initial onset) required for the ionosphere to transition from its initial state to its new state, which we refer to as the reconfiguration time, is measured. IMF Bz northward to southward transitions have reconfiguration times near local noon of approximately 5 minutes and lengthen approximately 2 minutes for every 3 hours of local time away from noon. ¿ 2001 American Geophysical Union