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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 29, issue 11
Ann. Geophys., 29, 2131–2146, 2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 29, 2131–2146, 2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.

Regular paper 28 Nov 2011

Regular paper | 28 Nov 2011

Cluster observations of a transient signature in the magnetotail: implications for the mode of reconnection

S. Beyene1, C. J. Owen1, A. P. Walsh1, C. Forsyth1, A. N. Fazakerley1, S. Kiehas2,3, I. Dandouras4,5, and E. Lucek6 S. Beyene et al.
  • 1Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, RH5 6NT, UK
  • 2Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
  • 3Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA
  • 4Institut de Recherche en Astrophysique et Planétologie (IRAP), University of Toulouse, UPS, 9 Ave. du Colonel Roche, B.P. 44346, 31028 Toulouse, France
  • 5IRAP, CNRS, B.P. 44346, 31028 Toulouse, France
  • 6Space and Atmospheric Physics, Imperial College London, SW7 2BZ, UK

Abstract. Travelling compression regions (TCRs) are perturbations in the magnetotail lobe magnetic field caused by structures moving Earthward or tailward within the plasma sheet. Previous works have suggested that these structures are created by either time-dependant reconnection occurring at a single X-line, forming a flux-bulge-type structure, or space-variant reconnection at multiple X-lines, forming flux-rope-type structures. In this study we examine an event in which Cluster 2 observed a TCR while the 3 remaining Cluster spacecraft observed the underlying magnetic structure at a range of distances from the neutral sheet. The magnetic structure has a velocity of (99, 154, −31) km s−1 in GSM (|V| = 186 km s−1), an estimated size of 1.19 RE along the direction of travel and a size between 1.94 and 2.86 RE in the direction perpendicular to the current sheet. As the structure passes the spacecraft, Cluster 1 and Cluster 4 observed a bipolar signature in BZ, plasma-sheet-like plasma and field-aligned electron flows. Cluster 3 passed closest to the centre of the structure and observed two separate reductions in the plasma density (with field-aligned electron flows); these drop-outs in the plasma sheet were possibly created by the actions of X-lines. The second drop-out in the plasma sheet also includes a reversal of the ion flow, a signature consistent with the passage of a reconnecting X-line past the spacecraft. Between the X-lines, the plasma outflow from the X-lines caused an increase in pressure which led to a localised expansion of the plasma and also the observations at Cluster 1 and Cluster 4 and the TCR. Our observations do not uniquely match either of the flux rope or the flux bulge predictions although the observation of two plasma sheet drop-outs (interpreted as X-lines, one active, one dormant) with plasma-sheet-like between them and only one TCR is a situation expected in multiple X-line reconnection.

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