Optical emissions and electorn flow patterns in the auroral oval evening secord have been investigated in the 30-min interval prior to a major auroral intensification. The STARE radar and Viking UV image data were combined to give a dynamical picture of the auroal ionosphere just to the west of the intensification region which was bordered on its eastward side by the foot of a high-latitude polar arc. Half an hour prior to the main intensification an auroal arc appeared equatorward of a band of eastward electrojet, which in turn was equatorward of another discrete are system. The appearance of the new arc system was accompanied by a second eastward electrojet further equatorward. Within minutes of this appearance the older electrojet and arc system faded and the new system moved poleward to replace it. This precursor activity was followed by a major intensification, a substorm, to the east of the foot of the high-latitude polar arc. It is speculated that the precursor is linked to changes in the current distribution in the near-Earth nightside magnetosphere prior to the substorm onset. ¿American Geophysical Union 1993