PARTICLE AND FIELD SIGNATURES DURING PSEUDOBREAKUP AND MAJOR EXPANSION ONSET

The temporal and spatial scales of the onset of two types of substorm events are investigated. These substorms were cases where the expansio n onset had precursor localized auroral activation without significant negative bay enhancement, that is, ''pseudobreakup''. High-resolution energetic particle and magnetic field data at synchronous orbit are u sed for the analysis together with auroral and magnetic field data sim ultaneously taken from ground-based instrumentation. The auroral struc ture following the pseudobreakup significantly resembled the major exp ansion aurora, except in its spatial scale. Typical magnetospheric ons et signatures such as tail current diversion, dipolarization, and inje ction were observed associated with some of the pseudobreakups. The ma jor expansion, on the other hand, consisted of a number of rather loca lized injection and expansions, each of which had timescales of 2-8 mi n, a comparable timescale to that of pseudobreakups. This study shows that there does not appear to be any phenomenological differences betw een pseudobreakups and major expansion onsets. The major difference be tween pseudobreakups and major expansion onsets would be the number of occurrences, as well as the intensity and the scale size of the magne tospheric source.