The Midlatitude Positive Bay Index and the Statistics of Substorm Occurrence

Abstract

AbstractThe auroral breakup and expansion are essential features of a magnetospheric substorm. Negative bays beneath the aurora and positive bays at midlatitudes accompany the expansion. These effects are caused by the substorm current wedge. The negative bay strength is provided by the auroral electrojet lower (AL) index. At midlatitudes the positive bay is caused by the field‐aligned currents connected to the electrojet. The midlatitude positive bay (MPB) index measures these effects. Both AL and MPB can be used to time auroral expansion onset. This work creates lists of MPB onsets and compares them with lists from other sources including onsets derived from IMAGE and Polar spacecraft and ground all‐sky camera data. The lists show that these measurements are nearly simultaneous with satellite expansion onsets. Substorms occur most frequently during the declining phase of the solar cycle, at the equinoxes, and after the passage of a stream interface in a corotating interaction region. Both MPB and SML indices reveal three substorm phases: growth, expansion, and recovery. In addition, the MPB onset list, the SML onset list, and the IMAGE auroral onset list reveal the presence of a 45‐min and a 2 3/4‐hr quasi‐periodicity in substorm occurrence. A comparison of the various lists reveals radical difference in their waiting time distributions and relatively low levels of association between events in the lists. The results suggest that none of the lists adequately capture the complexity of substorm activity and further refinements are needed in every algorithm used for onset detection.