Dynamics of the substorm expansive phase

Abstract

Meridian‐scanning photometer (MSP) and magnetometer data from the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) ground‐based array 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 emissions for eight isolated events have been studied. The data show the expansive phase to comprise three stages: an explosive (tens of seconds) onset; rapid poleward motion of 557.7 nm emissions of the order of a few minutes; and 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 accelerating plasma sheet electrons as it proceeds. All events show that lobe flux reconnection occurs after near‐Earth onset, with typical time delays between 1 and 5 min. The extremely short time interval between near‐Earth onset and the beginning of lobe flux reconnection presents a severe observational constraint on any substorm model which attempts to explain the substorm expansive phase. Magnetometer data from the CANOPUS Churchill line have been used to study the dynamics of the substorm electrojets during the growth and early expansive phases of the substorm. The borders of these electrojets closely follow the motion of the MSP data during the growth and expansive phases. At the instant of expansive phase onset the growth phase electrojets (as observed along the Churchill meridian line) disappear, being replaced by the substorm westward electrojet associated with the current wedge. The equatorward border of the substorm westward electrojet is embedded within the proton aurora emissions and appears at expansive phase onset ∼2° equatorward of the growth phase electrojets, further showing the near‐Earth proximity of expansive phase onset. No indication of reconnection on closed field lines prior to expansive phase onset was observed in either the magnetometer or the photometer data.