ON THE PLACE OF THE PSEUDO-BREAKUP IN A MAGNETOSPHERIC SUBSTORM

It is common to view the expansive phase of a magnetospheric substorm as a large amplitude disturbance. As. a result, any magnetic disturban ce that has the normal characteristics of an expansive phase onset but has an amplitude below some subjective limit is not classed as an exp ansive phase onset but is labelled, instead, as a pseudo-breakup. In t his paper it will be shown that a pseudo-breakup is, in fact, a distur bance which represents the same physical process as the onset of an ex pansive phase or any of the subsequent intensifications. Examination o f simultaneous interplanetary magnetic field data suggests the possibi lity that a pseudo-breakup is distinguished from the onset or intensif ications of an expansive phase by its occurrence during a period of in creasing energy input into the magnetosphere. Thus, the pseudo-breakup is not accompanied by a large scale poleward motion of the high latit ude edge of the auroral oval in the substorm disturbed region characte ristic of an expansive phase.