Substorms are among the most widely studied phenomena of space physics and remain among the most controversial. These spectacular releases of magnetospheric energy, which are manifested as spectacular dynamic auroral displays, have inspired much research and much passionate argument. The arguments center on the specific cause of substorm expansion onsets but extend to phenomenology. There is even controversy over the definition of a substorm such that some events would be labeled substorms by some researchers and pseudo breakups by other researchers.
One reason for controversy is the limited observational framework upon which theories must be based. As is true of most global-scale space physics phenomenon, it is not yet possible to observe all necessary parameters, simultaneously, at a sufficient number of locations, to uniquely identify the cause. With each new generation of spacecraft or ground-based instrumentation, however, we make progress toward providing the observational framework.
Recent studies of substorm convection patterns based upon SuperDARN data showed a characteristic pattern that appears to be repeatable. The studies focused on the growth phase and early expansion phase, but did not address expansion beyond the first few minutes. It was found that the growth phase is characterized by the enhancement of the velocity shear near midnight, and its extension to low latitudes and to local times across the midnight meridian. The velocity shear was observed to diminish at expansion onset. In addition the ionospheric flow velocity magnitude was observed to be enhanced for some period of time prior to expansion onset and to decrease at expansion onset.