We present two cases of abrupt dayside auroral brightenings and very fast auroral propagation using the POLAR UV imaging data. The brightenings occur first at noon and then propagate along the auroral oval towards dawn and dusk. Ionospheric speeds of 6 to 11 km/s are determined. The auroral brightenings and motion are associated with the arrival and propagation of interplanetary shocks/pressure waves. The brightening at noon occurs within minutes of the shock compression of the noon-time magnetopause. The speed of the auroral propagation in the ionosphere towards dawn and dusk corresponds extremely well to the solar wind downstream flow. Our model assumes that shocks/pressure waves compress the outer dayside magnetosphere, and plasma contained therein. This plasma compression leads to the loss cone instability, wave-particle interactions, and concomitant particle loss into the ionosphere. ¿ 1999 American Geophysical Union