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Ann. Geophys., 23, 2183-2198, 2005
www.ann-geophys.net/23/2183/2005/
© European Geosciences Union 2005
Transition from substorm growth to substorm expansion phase as observed with a radial configuration of ISTP and Cluster spacecraft
V. A. Sergeev1, M. V. Kubyshkina1, W. Baumjohann2, R. Nakamura2, O. Amm3, T. Pulkkinen3, V. Angelopoulos4, S. B. Mende4, B. Klecker5, T. Nagai6, J.-A. Sauvaud7, J. A. Slavin8, and M. F. Thomsen9
1St. Petersburg State University, Petrodvoretz, 198504 St. Petersburg, Russia
2Austrian Academy Science, Space Research Inst., Schmiedlstrasse 6, Graz, 8042, Austria
3Finnish Meteorological Inst., Helsinki, FIN 00101, Finland
4Univ California, Space Sciences Lab., 7 Gauss Way, Berkeley, CA 94720, USA
5Max Planck Institut für Extraterrestrische Physik, D-85741 Garching, Germany
6Tokyo Inst Technology., Dept Earth & Planetary Sciences., Ookayama 2-12-1 Meguro., Tokyo, 152-8551, Japan
7CESR, 9 Ave Colonel Roche, Toulouse Cedex 4, 31028, France
8NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
9Los Alamos Natl Lab., MS D466, Los Alamos, NM 87545, USA
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)
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