Observations from the Sweden and Britain Radar Experiment and the Scandinavian Twin Auroral Radar Experiment are presented of a Pc 5 pulsation which was monochromatic in a reference frame moving in north-south direction with respect to the radar grid. This reference frame was fixed with respect to a boundary across which the latitudinal phase variation of the pulsation experienced a distinct discontinuity. The pulsation was embedded in westward or convection flow, and its azimuthal wave number was low. The data are reminiscent of those of a double resonance but nevertheless differ in significant ways. The event is discussed in terms of recent theoretical developments which, however, are not found to provide an adequate explanation. ¿ American Geophysical Union 1988