What is Cluster telling us about magnetotail dynamics?

In this review, we report on some new aspects of magnetotail dynamics found in the data of the first traversal of the magnetotail by the Cluster quartet in summer and autumn 2001: (1) The electron drift instrument made the first direct measurements of tail lobe convection. The statistical data shows convection toward the center of the plasma sheet, with a clear dependence on the sign of the interplanetary magnetic field BZ component. Moreover, a dawn-dusk shear (if one compares convection in opposite lobes) for BY-dominated interplanetary field hints to an interconnection of open lobe field lines with the interplanetary medium. (2) At times the tail current sheet resembles a one-dimensional Harris sheet, which might get as thin as 500 km and may carry current densities as high as 20-40 nA/m2. (3) At other times, the current sheet may exhibit rapid kink-type flapping motion with vertical velocities of 50-100 km/s. During these intervals the current sheet clearly exhibits a bifurcated structure, with two current density maxima around a region of much reduced current in the center of the plasma sheet.