DEVELOPMENT OF SUBSTORM CROSS-TAIL CURRENT DISRUPTION AS SEEN FROM THE GROUND

We discuss substorm observations made near 2100 magnetic local time (M LT) on March 7, 1991, in a collaborative study involving data from the European Incoherent Scatter radar, all-sky camera data, and magnetome ter data from the Tromso Auroral Observatory, the U.K. Sub-Auroral Mag netometer Network (SAMNET) and the IMAGE magnetometer chain. We conclu de that for the substorm studied a plasmoid was not pinched off until at least 10 min after onset-at the local time of the observations (210 0 MLT) and that the main substorm electrojet expanded westward over th is local time 14 min after onset. In the late growth phase/early expan sion phase, we observed southward drifting arcs probably moving faster than the background plasma. Similar southward moving arcs in the reco very phase moved at a speed which does not appear to be significantly different from the measured plasma flow speed. We discuss these data i n terms of the ''Kiruna conjecture'' and classical ''near-Earth neutra l line'' paradigms, since the data show features of both models of sub storm development. We suggest that longitudinal variation in behavior may reconcile the differences between the two models in the case of th is substorm.