During two consecutive orbits of the Viking satellite on March 24, 1986, UV observations of the northern hemisphere auroral distribution revealed rapid growth and decay of large-scale polar arcs. Evolution of these features occurred from the nightside auroral distribution (to which they are optically connected) toward the dayside. The connection on the dayside was short-lived (≈2 min) and the arc retreated at similar speeds to its development (≈ 5 km/s). Time scales for growth (at least to the level of the sensitivity of the instrument) can also be less than 1 min. Examples of arc occurrence during a half-hour time period show that arcs can extend from the nightside to the dayside and disappear and another extended arc can appear at a widely separated position. These types of dynamic polar features appear consistent with the dynamic energization and precipitation of boundary layer electrons at high latitudes. ¿ American Geophysical Union 1989