23 Jun 2004

Animation 1: Full orbit of the Cluster space fleet over a simulated Earth's magnetosphere responding to the Sun's activity (Copyright: ESA)

Rapid large-amplitude magnetic variations (Image 1) are frequently observed in the centre of the magnetotail. These variations indicate up and down (north-south) oscillating current sheet motions, known as current sheet flapping motions. Two possible origins of these motions have been proposed in the past: external (inhomogeneous solar wind) or internal (large-scale wave, emitted in the current sheet and/or propagating in the tail).

Image 1: Overview of Cluster observations on 20 October 2001, including: Ion plasma flows from CIS-CODIF (a, b) ; Bx-components at spacecraft C1, C2 and C3 (e) ; Magnetic field x- and z- component computed at the Cluster baricenter (c) ; xB estimates by curlometer technique (panel d, only samples with |·B|/|xB| < 0.2 are included) [Sergeev et al., 2004].

"Previous studies [...] suggested that substorms* play some role in the excitation of the flapping waves", reminds Prof. Victor Sergeev, lead author on this research, performed in close collaboration with the Space Research Institute /Austrian Academy of Sciences, Graz, and the Centre d'Etude Spatiale des Rayonnements (CNRS/Université Paul Sabatier), Toulouse, published 06 March 2004, in Geophysical Research Letters.

"This new finding not only supports this view, it also provides some constraints to the (yet) unknown mechanisms which destabilize the tail sheet in the substorm process and launch the kink waves", he added. (Image 2).

Image 2: Interpretation scheme of kink waves launched during a substorm process (courtesy of Prof. V. Sergeev).

"Finding bifurcated current sheets (BCSs) in the Earth's magnetotail and the detailed description of their 3D spatial structure and flapping motions has become one of the most significant accomplishments of the unique four-spacecraft Cluster mission", underlined Dr. Mikhail Sitnov in a recent paper, published 8 May 2004 in Geophysical Research Letters.

Image 3: 3D numerical simulation of the 4 Cluster satellites embedded in the flapping motion of the current sheet (courtesy of Dr. J. Buechner, Max Planck Institut für Aeronomie, Katlenburg-Lindau, Germany).

* Substorms: violent reconfigurations of the Earth's magnetic tail which, by change in the magnetic topology, convert magnetic energy to energize ions and electrons, inject some of them into the ring current, and greatly increase the rate at which energy is released in the magnetosphere.

Sergeev, V., A. Runov, W. Baumjohann, R. Nakamura, T.L. Zhang, A. Balogh, P. Louarn, J.-A. Sauvaud, and H. Rème (2004), Orientation and propagation of current sheet oscillations, Geophys. Res. Lett., 31, L05807, doi:10.1029/2003GL019346, 2004.

Related articles

Nakamura, R., W. Baumjohann, A. Runov, M. Volwerk, T.L. Zhang, B. Klecker, Y. Bogdanova, A. Roux, A. Balogh, H. Rème, J.A. Sauvaud, H.U. Frey: Fast Flow during current sheet thinning. Geophys. Res. Lett., 29, 2140, doi:10.1029/2002GL016200, 2002

Runov, A., R. Nakamura, W. Baumjohann, T. L. Zhang, M. Volwerk, and H.-U. Eichelberger (2003), Cluster observation of a bifurcated current sheet, Geophys. Res. Lett., 30(2), 1036, doi:10.1029/2002GL016136, 2003.

Sitnov, M. I., M. Swisdak, J. F. Drake, and P.N. Guzdar (2004), A model of the bifurcated current sheet: 2. Flapping motions, Geophys. Res. Lett., 31, L09805, doi:10.1029/2004GL019473, 2004.

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