A sharp latitudinal gradient in the Doppler spectral width is often observed in the nightside data from the SuperDARN HF radars. We use an excellent conjunction between the Halley HF radar and the POLAR satellite to demonstrate that this gradient is associated with the boundary between the central plasma sheet and boundary layer plasma precipitation. We then exploit this identification to examine the CPS/BPS response simultaneously near the dawn, dusk and midnight, to a significant change of IMF By during a period when the magnetosphere is otherwise in a quiescent state. We find that the direction of the boundary motions are consistent with those previously determined by Holzworth and Meng, [1984>. In the morning sector, the magnitude is consistent with previous observations but in the evening sector the displacement is about 3 times greater than expected. Also the response appears to be in the order of minutes near dawn and dusk, faster than previously reported responses to IMF changes in this region. ¿ 1998 American Geophysical Union