Transition from Substorm Growth to Substorm Expansion Phase as Observed with a Radial Configuration of ISTP and Cluster Spacecraft

Details Transition from Substorm Growth to Substorm Expansion Phase as Observed with a Radial Configuration of ISTP and Cluster Spacecraft Transition from Substorm Growth to Substorm Expansion Phase as Observed with a Radial Configuration of ISTP and Cluster Spacecraft

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufmsm21b..05s&link_type=abstract

American Geophysical Union, Fall Meeting 2004, abstract #SM21B-05

Physics

2407 Auroral Ionosphere (2704), 2740 Magnetospheric Configuration And Dynamics, 2764 Plasma Sheet, 2788 Storms And Substorms

Scientific paper

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 spacecraft, and of the inner region at 6.6 Re with two LANL spacecraft. The activity development in both global scale and near the spacecraft footpoints was controlled with global auroral images (from the IMAGE spacecraft) and the ground network. The 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 the local B minimum in the neutral sheet (~5nT) at ~10-12 Re near the substorm onset. However, the time delays around the distinct substorm onset show the propagation from the midtail to the inner magnetosphere suggesting that this strongly enhanced current was not spontaneously disrupted. The earliest perturbations near the onset observed at Cluster just outside the thinned plasma sheet are interpreted as the being due to localized Earthward-contracting newly-reconnected plasma tubes produced by the impulsive reconnection in the midtail plasma sheet. A peculiar feature of this substorm was that 12 min 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 ~9Re. This pseudobreakup was probably due to either localized ballooning-type activity or due to the braking of very narrow BBF whose signatures were also recorded by the 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 both are 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.

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