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

V. Sergeev, M. Kubyshkina, W. Baumjohann, R. Nakamura, O. Amm, T. Pulkkinen, 
V. Angelopoulos, S. Mende, B. Klecker, T. Nagai, J. Sauvaud, J. Slavin, 
and M. Thomsen


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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Presentation, Fall A.G.U. Meeting, San Francisco, CA, 13-17 December 2004