The flapping motions of the Earth�s plasma sheet are of internal origin, according to a large set of observations from the Cluster satellites. This dataset includes 58 rapid plasma sheet crossings, observed by Cluster near the magnetotail flanks.

Rapid large-amplitude magnetic variations 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).

Before Cluster, no final conclusion could be drawn, due to the difficulties of separating spatial and temporal effects. This time, a systematic analysis of tens of rapid plasma sheet crossings suggests that the flapping motions are of internal origin.

Waves, called kink-like waves, are emitted in the centre of the tail and propagate toward the tail flanks. The properties of these waves (phase velocity, direction of propagation) differ from any local excitation mechanism, discussed so far in the literature. The FluxGate Magnetometer (PI: Prof. Andr� Balogh) has been the primary experiment used for this data analysis.

"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/Universit� 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.

This new result is far from being the only one found by Cluster in the Earth�s magnetotail. In particular, Cluster has provided the first observational evidence of a very thin current sheet (few thermal ion gyroradii) [Nakamura et al., 2002], at times split into two layers, also called bifurcated current sheets [Runov et al., 2003]. In the latter case study, the two current sheets were also vertically flapping in the north-south direction.

"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.

Due to these new Cluster findings, this paper presents numerical simulation results, modeling the behaviour of the current sheet flapping motions. And there is more to come: "the new observations [performed by Cluster] triggered a burst of modeling activity", he added.

* 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.

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