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Ann. Geophys., 24, 603-618, 2006
www.ann-geophys.net/24/603/2006/
© European Geosciences Union 2006
The structure of flux transfer events recovered from Cluster data
H. Hasegawa1, B. U. Ö. Sonnerup2, C. J. Owen3, B. Klecker4, G. Paschmann4, A. Balogh5, and H. Rème6
1Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, Japan
2Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire, USA
3Mullard Space Science Laboratory, University College London, Dorking, Surrey, UK
4Max-Planck-Institut für extraterrestrische Physik, Garching, Germany
5Space and Atmospheric Physics Group, Imperial College, London, UK
6Centre d’Etude Spatiale des Rayonnements, Toulouse, France
Abstract. The structure and formation mechanism of a total of five Flux Transfer Events
(FTEs), encountered on the equatorward side of the northern cusp by the
Cluster spacecraft, with separation of ~5000 km, are studied by
applying the Grad-Shafranov (GS) reconstruction technique to the events. The
technique generates a magnetic field/plasma map of the FTE cross section,
using combined magnetic field and plasma data from all four spacecraft, under
the assumption that the structure is two-dimensional (2-D) and
time-independent. The reconstructed FTEs consist of one or more magnetic flux
ropes embedded in the magnetopause, suggesting that multiple X-line
reconnection was involved in generating the observed FTEs. The dimension of
the flux ropes in the direction normal to the magnetopause ranges from about
2000 km to more than 1 RE. The orientation of the flux rope
axis can be determined through optimization of the GS map, the result being
consistent with those from various single-spacecraft methods. Thanks to this,
the unambiguous presence of a strong core field is confirmed, providing
evidence for component merging. The amount of magnetic flux contained within
each flux rope is calculated from the map and, by dividing it by the time
interval between the preceding FTE and the one reconstructed, a lower limit
of the reconnection electric field during the creation of the flux rope can
be estimated; the estimated value ranges from ~0.11 to ~0.26
mV m-1, with an average of 0.19 mV m-1. This can be
translated to the reconnection rate of 0.038 to 0.074, with an average of
0.056. Based on the success of the 2-D model in recovering the observed FTEs,
the length of the X-lines is estimated to be at least a few RE.
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