  We have determined the spatiotemporal characteristics of the
magnetosphere-ionosphere (M-I) coupling using auroral imaging. Observations at
fixed positions for an extended period of time are provided by a ground-based
all-sky imager measuring the 557.7 nm auroral emissions. We report on a single
event of nightside aurora ($\sim$22 magnetic local time) preceding a substorm
onset. To determine the spatiotemporal characteristics, we perform an
innovative analysis of an all-sky imager movie (19 min duration, images at 3.31
Hz) that combines a two-dimensional spatial fast Fourier transform with a
temporal correlation. We find a scale size-dependent variability where the
largest scale sizes are stable on timescales of minutes while the small scale
sizes are more variable. When comparing two smaller time intervals of different
types of auroral displays, we find a variation in their characteristics. The
characteristics averaged over the event are in remarkable agreement with the
spatiotemporal characteristics of the nightside field-aligned currents during
moderately disturbed times. Thus, two different electrodynamical parameters of
the M-I coupling show similar behavior. This gives independent support to the
claim of a system behavior that uses repeatable solutions to transfer energy
and momentum from the magnetosphere to the ionosphere.

Space Physics