SIMULTANEOUS SUPERDARN AND CLUSTER OBSERVATIONS OF THE GROWTH AND EXPANSION PHASES OF A SUBSTORM

A coordinated study of a substorm that occurred on 5 October 2002 is presented using instruments located at ~15 RE downtail, at a near-Earth location of 6.6 RE downtail and on Earth. Cross-tail currents are detected by the Cluster spacecraft at ~15 RE downtail from 15 min before onset until 3 min before onset. Near-Earth substorm signatures are detected from one minute after expansion phase onset, determined by the detection of electrojet and substorm current wedge signatures in the CANOPUS magnetometer data. Subsequently, 12 min after onset the Cluster spacecraft detect first the dawnward edge, then 40 min later the duskward edge, of the substorm current wedge (SCW), indicating that this has moved dawnward during the expansion phase. The corresponding footprint of the SCW is inferred from magnetometer data available from CANOPUS and Greenland magnetometer data. The SuperDARN radars reveal an intensification in ionospheric flow 22 min after substorm onset. We conclude that a cross-tail current sheet is detected at the orbit of the Cluster spacecraft, the signature of which is lost 3 minutes prior to substorm onset, perhaps due to thinning of the current sheet