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Transition from the growth phase to the substorm expansion during awell-isolated substorm with a strong growth phase is investigatedusing a unique radial (THEMIS-like) spacecraft constellation nearmidnight, including the probing of the tail current at ~16 RE with Cluster,of the transition region at ~9 RE withGeotail and Polar, and of the inner region at 6.6 RE with twoLANL spacecraft. The activity development on both a global scale andnear the spacecraft footpoints was monitored with global auroralimages (from the IMAGE spacecraft) and the ground network.Magnetospheric models, tuned using in-situ observations, indicateda strong tail stretching and plasma sheet thinning, which includedthe growth of the near-Earth current (approaching 30 nA/m2) andpossible formation of a local B minimum in the neutral sheet(~5 nT) at ~10–12 RE near the substorm onset.However, there were no indications that the substorm onset wasinitiated just in this region. We emphasize the rather weakmagnetic and plasma flow perturbations observed outside thethinned plasma sheet at Cluster, which could be interpreted as theeffects of localized earthward-contracting newly-reconnectedplasma tubes produced by the impulsive reconnection in the midtailplasma sheet. In that case the time delays around the distinctsubstorm onset are consistent with the activity propagation fromthe midtail to the inner magnetosphere. A peculiar feature ofthis substorm was that 12min prior to this distinct onset, aclear soft plasma injection to the GEO orbit was recorded whichhas little associated effects both in the ionosphere and in thetransition region at ~9 RE. This pseudo-breakup wasprobably due to either a localized ballooning-type activity or dueto the braking of a very narrow BBF whose signatures were alsorecorded by Cluster. This event manifested the (previouslyunknown) phenomenon, a strong tail overloading (excessive storageof magnetic energy) contrasted to the modest energy dissipationand plasma acceleration, which are both discussed and interpretedas the consequences of cold/dense and thick pre-substorm plasmasheet which often occurs after the long quiet period. The lessonsof using the radial spacecraft configurations in substorm onsetstudies are also discussed. Keywords. Magnetospheric physics (Auroral phenomena,plasma sheet, storms and substorms

Transition from substorm growth to substorm expansion phase as observed with a radial configuration of ISTP and Cluster spacecraft