Dynamics of the substorm expansive phase

Meridian-scanning photometer (MSP) and magnetometer data from the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) ground-based a rray have been used to study the dynamics of the substorm expansive phase. The relative latitudinal motions of the MSP 630.0, 557.7. and 486.1 nm emis sions for eight isolated events have been studied. The data show the expans ive phase to comprise three stages: an explosive (tens of seconds) onset; r apid poleward motion of 557.7 nm emissions of the order of a few minutes; a nd a period of slower (tens of minutes) poleward moving 630.0 nm emissions. We interpret the rapid poleward motion of the 557.7 nm data in terms of a region of instability, expanding rapidly down the magnetotail, possibly acc elerating plasma sheet electrons as it proceeds. All events show that lobe flux reconnection occurs after near-Earth onset, with typical time delays b etween 1 and 5 min. The extremely short time interval between near-Earth on set and the beginning of lobe flux reconnection presents a severe observati onal constraint on any substorm model which attempts to explain the substor m expansive phase. Magnetometer data from the CANOPUS Churchill line have b een used to study the dynamics of the substorm electrojets during the growt h and early expansive phases of the substorm. The borders of these electroj ets closely follow the motion of the MSP data during the growth and expansi ve phases. At the instant of expansive phase onset the growth phase electro jets (as observed along the Churchill meridian line) disappear, being repla ced by the substorm westward electrojet associated with the current wedge. The equatorward border of the substorm westward electrojet is embedded with in the proton aurora emissions and appears at expansive phase onset similar to2 degrees equatorward of the growth phase electrojets, further showing t he near-Earth proximity of expansive phase onset. No indication of reconnec tion on closed field lines prior to expansive phase onset was observed in e ither the magnetometer or the photometer data.