Substorms: Fundamental observational features, distinction from other disturbances, and external triggering

Abstract

Fundamental observational signatures of substorms and other auroral zone disturbances are summarized in this review. It is shown that at least three distinct types of major geomagnetic disturbance have been identified in the auroral zones, and it is suggested that care should be taken in distinguishing substorms from the other types of disturbance when reaching observational conclusions concerning substorms. Substorms can be uniquely identified by the formation of the current wedge and by the associated westward electrojet and region of active aurora that move poleward within the ionosphere during the expansion phase. Periods of enhanced convection have been observed to continue without substorms for as long as the interplanetary magnetic field (IMF) remains stably southward. They can have AL signatures that look very much like intense substorms, but these signatures result from strong DP 2 ionospheric currents driven by enhanced convection rather than from a nightside, westward electrojet. A third type of disturbance is characterized by short intervals of enhanced equatorward flow within the ionosphere and enhanced auroral emissions. These enhancements move equatorward from near the polar cap boundary, and they may be associated with several‐minute‐long enhancements of earthward flow within the plasma sheet. Studies that have examined the triggering of substorm expansions by IMF changes are also reviewed. When considered together, and substorms are distinguished from the other disturbances, these studies imply that most, and perhaps all, expansions are triggered by IMF changes. These changes include both northward turnings and reductions in the magnitude of the y component. If most substorms are indeed externally triggered as suggested here, then substorms must generally not be the result of an internal magnetospheric instability.