We have examined the dayside high-latitude convection response to a sudden southward turning of the Interplanetary Magnetic Field (IMF) and found that the response is nearly instantaneous (<2 min) over a spatial region extending from ~75¿ to 85¿ and from ~9 to 16 MLT. Observations of the magnetic field were made with the WIND spacecraft in the solar wind and the GEOTAIL and IMP8 spacecraft in the magnetosheath. In the high-latitude ionosphere, the HF radars of the Super Dual Auroral Radar Network (SuperDARN) were used to monitor the convection. Based on the magnetosheath flow of the gas dynamic approximation, the field lines of the new IMF state were draped over a large portion of the dayside magnetopause when the first significant indication of convection response was measured in the ionosphere. Significant magnetic field line draping accompanied by extended reconnection on the dayside magnetopause may help explain the rapid, large-scale response of ionospheric convection. ¿ 1999 American Geophysical Union