Combined observations by meridian-scanning photometers and the EISCAT radar show that the ''midday-auroral breakup'' phenomenon is associated with major increases in ionospheric flow. A sequence of nine events is observed in the early afternoon MLT sector during a period when the IMF is strongly southward with a large positive By component. Each auroral structure is seen at both 630 and 557.7 nm and initially moves westward, accompanied by an increase in potential of 30--60 kV across the north-south dimension of the EISCAT field-of-view. After a few minutes the arc (or arc fragment) moves into the polar cap and fades, and the velocities observed by the radar swing from westward toward northward. We conclude that dayside auroral breakup is closely associated with momentum transfer across the magnetopause which occurs in a series of events 5--15 minutes apart. The largest of the observed events has dimensions of about 300 km (in the direction of westward motion) by 700 km, is bounded on its poleward edge by a 5kR arc and is associated with a potential of at least 80 kV. ¿ American Geophysical Union 1989