Transition from substorm growth to substorm expansion phase as observed with a radial configuration of ISTP and Cluster spacecraft
Abstract. Transition from the growth phase to the substorm expansion during a well-isolated substorm with a strong growth phase is investigated using a unique radial (THEMIS-like) spacecraft constellation near midnight, including the probing of the tail current at ~16 RE with Cluster, of the transition region at ~9 RE with Geotail and Polar, and of the inner region at 6.6 RE with two LANL spacecraft. The activity development on both a global scale and near the spacecraft footpoints was monitored with global auroral images (from the IMAGE spacecraft) and the ground network. Magnetospheric models, tuned using in-situ observations, indicated a strong tail stretching and plasma sheet thinning, which included the growth of the near-Earth current (approaching 30 nA/m2) and possible 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 was initiated just in this region. We emphasize the rather weak magnetic and plasma flow perturbations observed outside the thinned plasma sheet at Cluster, which could be interpreted as the effects of localized earthward-contracting newly-reconnected plasma tubes produced by the impulsive reconnection in the midtail plasma sheet. In that case the time delays around the distinct substorm onset are consistent with the activity propagation from the midtail to the inner magnetosphere. A peculiar feature of this substorm was that 12min prior to this distinct onset, a clear soft plasma injection to the GEO orbit was recorded which has little associated effects both in the ionosphere and in the transition region at ~9 RE. This pseudo-breakup was probably due to either a localized ballooning-type activity or due to the braking of a very narrow BBF whose signatures were also recorded by Cluster. This event manifested the (previously unknown) phenomenon, a strong tail overloading (excessive storage of magnetic energy) contrasted to the modest energy dissipation and plasma acceleration, which are both discussed and interpreted as the consequences of cold/dense and thick pre-substorm plasma sheet which often occurs after the long quiet period. The lessons of using the radial spacecraft configurations in substorm onset studies are also discussed.
Keywords. Magnetospheric physics (Auroral phenomena, plasma sheet, storms and substorms)