Cluster observation of a bifurcated current sheet

Abstract

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.