In August 2001,Cluster satellites observed that the mid-tail current sheet(CS) moved southward continuously for almost seven hours.Meanwhile,Cluster crossed back and forth the CS repeatedly.This means that the large-scale southward movement of the CS was accompanied by a small-scale CS flapping during this period.Using the minimum-variation-analysis(MVA) method and the multi-spacecraft data,we calculated the normal vector,current density and the magnetic curvature of the CS,the results showed that the CS alternated between flattened CS and tilted CS for several times.Strong dawn-dusk oscillations were found for the tilted CS,which caused the repeated crossings of the center of CS by the satellites.This feature is obviously different from the previous observations of the vertical flapping of the CS induced by the kink instability.Two types of flapping were observed:One of them is accompanied with bursty bulk flows(BBFs) and the other is not.This suggests that in this event there was no direct relationship between the CS flapping and BBFs.
In this paper,the y-component of magnetic field line curvature in the plasma sheet was analyzed,and two kinds of shear structures of the flapping current sheet were found,i.e.symmetric and antisymmetric.The alternating bending orientations of the guiding field are exactly corresponding to alternating north-south asymmetries of the bouncing ion population in the sheet center.Those alternating asymmetric plasma sources consequently induce the current sheet flapping motion as a driver.In addition,a substantial particle population with downward motion was observed in the center of a bifurcated current sheet.This population is identified as the quasi-adiabatic particles,and provides a net current opposite to the conventional cross-tail current.
