Articles | Volume 25, issue 3
Ann. Geophys., 25, 801–814, 2007
Ann. Geophys., 25, 801–814, 2007

  29 Mar 2007

29 Mar 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 S. V. Apatenkov et al.
  • 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.