Time-dependent model studies of the aurora show that various ionospheric parameters respond to the onset of auroral ionization with different time histories. While satellite-borne instruments sample the spatial morphology of these parameters, geostationary detectors are needed to follow the temporal morphology at a point in space. A pass of the Atmosphere Explorer C satellite over Poker Flat, Alaska, and ground-based photometric and photographic observations have been used to resolve the time-space ambiguity of a specific auroral event. The density of the O+, NO+, and N+2 ions, the electron density, and the electron temperature observed at 280--km altitude in a 50 km wide segment of an auroral arc are predicted by the model if particle precipitation into the region commenced about 11 min prior to the overpass. Continuous photometric measurements and all-sky photographs taken at 1-min intervals from Poker Flat show that the arc formed and became bright approximately 11 min before the satellite measurements were made.