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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 23, issue 12
Ann. Geophys., 23, 3667-3683, 2005
https://doi.org/10.5194/angeo-23-3667-2005
© Author(s) 2005. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 23, 3667-3683, 2005
https://doi.org/10.5194/angeo-23-3667-2005
© Author(s) 2005. This work is distributed under
the Creative Commons Attribution 3.0 License.

  23 Dec 2005

23 Dec 2005

Cluster magnetotail observations of a tailward-travelling plasmoid at substorm expansion phase onset and field aligned currents in the plasma sheet boundary layer

N. C. Draper1, M. Lester1, S. W. H. Cowley1, J.A. Wild1, S. E. Milan1, G. Provan1, A. Grocott1, A. N. Fazakerley2, A. Lahiff2, J. A. Davies3, J.-M. Bosqued4, J. P. Dewhurst2, R. Nakamura5, C. J. Owen2, J. Waterman6, M. G. Henderson7, H. J. Singer8, and E. Donovan9 N. C. Draper et al.
  • 1Radio and Space Plasma Physics Group, University of Leicester, University Road, Leicester, LE1 7RH, UK
  • 2Mullard Space Science Laboratory, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
  • 3Space Science and Technology Department, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK
  • 4CESR/CNRS, BP 4346 9, Avenue Colonel Roche, 31028 Toulouse CEDEX 4, France
  • 5Space Research Institute, Austrian Academy of Sciences, 8042 Graz, Austria
  • 6Danish Meteorological Institute, Copenhagen, Denmark
  • 7Los Alamos National Laboratory, Los Alamos, NM 87545, USA
  • 8Space Environment Center – National Oceanic and Atmospheric Administration, Boulder, CO 80305, USA
  • 9University of Calgary, 2500 University Drive NW, Calgary, Alberta, T2N 1N4, Canada

Abstract. We present data from both ground- and space-based instruments for a substorm event which occurred on 5 October 2002, with an expansion phase onset time of 02:50 UT determined from the ground magnetometer data. During this substorm, the Cluster spacecraft were located around 15 RE downtail, 8 RE from midnight in the pre-midnight sector and just 2 RE above the equatorial plane (in GSM coordinates). At expansion phase onset the Cluster spacecraft were located in the plasma sheet, tailward of a near-Earth neutral line and detected a significant time delay of 6 min between the tail field Bz component becoming negative and the subsequent detection of Earthward flows. This is explained by the formation of a tailward-directed travelling compression region initially Earthward of the spacecraft; 7 min later the Cluster spacecraft entered the plasma sheet boundary layer; they remained in and close to the plasma sheet boundary layer for around 15 min before exiting to the lobe. The spacecraft then re-entered the plasma sheet 30 min after onset. Earthward then tailward directed currents detected in the plasma sheet boundary layer after onset indicate that the Cluster spacecraft encountered the dawnward and duskward portions of the reconnection flow associated current system with Region 1 sense, respectively. The reconnection site and current system were initially skewed towards the pre-midnight sector, consistent with previous observations that found the majority of substorm onsets located in this sector. At later times the reconnection site and current system had moved towards dawn, to be located more centrally in the midnight sector.

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