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Ann. Geophys., 25, 801-814, 2007
www.ann-geophys.net/25/801/2007/
© European Geosciences Union 2007
Multi-spacecraft observation of plasma dipolarization/injection in the inner magnetosphere
S. V. Apatenkov1, V. A. Sergeev1, M. V. Kubyshkina1, R. Nakamura2, W. Baumjohann2, A. Runov2, I. Alexeev3, A. Fazakerley3, H. Frey4, S. Muhlbachler5, P. W. Daly5, J.-A. Sauvaud6, N. Ganushkina7, T. Pulkkinen7, G. D. Reeves8, and Y. Khotyaintsev9
1Institute of Physics, St. Petersburg State University, St. Petersburg, Russia
2Space Research Institute, Austrian Academy of Sciences, Graz, Austria
3Mullard Space Science Laboratory, University College London, UK
4Space Science Laboratory, Berkeley, USA
5Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany
6Centre d'Etude Spatiale des Rayonnements, Toulouse, France
7Finnish Meteorological Institute, Helsinki, Finland
8Los Alamos National Laboratory, USA
9Swedish Institute of Space Physics, Uppsala, Sweden
Abstract. Addressing the origin of the energetic particle injections into the inner
magnetosphere, we investigate the 23 February 2004 substorm using a favorable
constellation of four Cluster (near perigee), LANL and Geotail spacecraft.
Both an energy-dispersed and a dispersionless injection were observed by
Cluster crossing the plasma sheet horn, which mapped to 9–12 RE in the
equatorial plane close to the midnight meridian. Two associated narrow
equatorward auroral tongues/streamers propagating from the oval poleward
boundary could be discerned in the global images obtained by IMAGE/WIC. As
compared to the energy-dispersed event, the dispersionless injection front
has important distinctions consequently repeated at 4 spacecraft:
a simultaneous increase in electron fluxes at energies ~1..300 keV,
~25 nT increase in BZ and a local increase by a factor 1.5–1.7 in
plasma pressure. The injected plasma was primarily of solar wind origin. We
evaluated the change in the injected flux tube configuration during the
dipolarization by fitting flux increases observed by the PEACE and RAPID
instruments, assuming adiabatic heating and the Liouville theorem. Mapping the
locations of the injection front detected by the four spacecraft to the
equatorial plane, we estimated the injection front thickness to be ~1 RE
and the earthward propagation speed to be ~200–400 km/s (at 9–12 RE).
Based on observed injection properties, we suggest that it is
the underpopulated flux tubes (bubbles with enhanced magnetic field and sharp
inner front propagating earthward), which accelerate and transport particles into
the strong-field dipolar region.
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