Current sheet flapping motion and structure observed by Cluster

Fast vertical flapping oscillations of the plasma sheet have been observed by Cluster on September 26, 2001. The flapping motion had vertical speeds exceeding 100 km/s, an amplitude in excess of 1 R E and a quasiperiod of ∼3 min. The current sheet was mostly tilted in the Y ‐ Z plane (with the tilt sometimes exceeding 45°). The waves had the properties of a kink mode and propagated toward the dusk flank. The flapping allowed to probe the vertical structure of the plasma sheet. Three different methods gave consistent evidence of a bifurcated structure of the cross‐tail current with about half of all current concentrated in two sheets (each ∼500–1000 km thick). The current density peaks at ∣ B x ∣ ∼ 0.5 B L , with a pronounced current density minimum and a plasma density plateau between these peaks.