A special SPA: Magnetospheric Physics session dedicated to the recovery phase of substorms is being planned for the AGU 1992 Fall Meeting. Substorms have provided an important focus for magnetospheric research for nearly 3 decades, and continue to do so today. Of the three phases (growth, expansive, and recovery) that constitute a substorm, the growth and expansive phases have received the most attention, and a fairly good understanding of them has developed.
In contrast, the recovery phase has received scant attention. Perhaps that is because it lacks the dramatic auroral and geomagnetic effects that mark the expansive phase at Earth and the similarly impressive particle and magnetic field effects that mark the growth phase to expansive phase transition at geosynchronous orbit. The recovery phase was originally defined as the period when auroras, having advanced far toward the pole during the expansive phase, become less intense and retreat back toward the auroral oval. Wavy structures—omega bands—and pulsating intensities often characterize the recovery phase auroras. It has become customary, also, to define the recovery phase as that period when the Auroral Electrojet (AE) index is subsiding from the peak value it reached during the substorm. These two definitions, probably not altogether compatible, may identify somewhat different periods of a substorm. There is no obvious feature in geosynchronous observations that has been specifically identified with the recovery phase.