Cluster observation of a bifurcated current sheet

The current sheet structure and motion at X GSM = −19.5 R E , observed by Cluster/FGM during 1055–1107 UT on 29 August 2001, is examined. It is found that during the interval 1055–1102 UT the current sheet moves vertically up and down with a velocity of about 60 km/s. 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. In the interval 1103–1107 UT the current sheet moves slowly upward and the current sheet has a Harris‐type structure. By 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.