ESA Science & Technology11-Jul-2005 14:59:32

Science Results

Bifurcation of the tail current

X component of the magnetic field measured on the four spacecraft (C1 in black, C2 in blue, C3 in green and C4 in red) on 29 August 2001. The shematic view of the current bifurcation is show on the bottom panel (From Runov et al., Geophys. Res. Lett., 2003b).

The cross-magnetotail current sheet, which separates the northern lobe of the magnetotail from the southern lobe, is one of the key objects of magnetospheric physics.

In the simplest 1-D approximation the current sheet has the maximum value in the neutral sheet. However sometimes, the current sheet is perturbed and it can have a maximum on both side of the neutral sheet. Previous observations from ISEE (at around 11 Earth radii) and recently Geotail (at 100 Earth radii) have observed bifurcated current sheets. With Cluster we can confirm that the bifurcation is also observed at 20 Earth radii. Furthermore the speed of the current sheet could also be measured and a wave was also observed propagating simultaneously in the dawn-dusk direction.

The current sheet structure and motion was observed by Cluster/FGM on 29 August 2001. It was found that between 1055 and 1102 UT the current sheet moves vertically up and down with a velocity of about 60 kms-1. During this interval the current sheet has a bifurcated structure: electric current is concentrated in two sheets with an extended layer of weak nearly uniform magnetic field in between. After that time, between 1103 and 1107 UT the current sheet moves slowly upward and the current sheet has no bifurcation anymore. Using four-spacecraft timing analysis, it is shown that the fast motion and bifurcation of the current sheet are associated with a wave-like transient propagating in the dawn-to-dusk direction. This wave had a size of 2000 km and was propagating at 200 kms-1. The physical mechanism producing the motion and bifurcation of the current sheet is not known yet. Magnetic reconnection was suggested but is not yet adopted. These results were published in Runov et al., Geophys. Res. Lett., 2003a).

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