In this paper we study a flow burst event which took place during enhanced geomagnetic activity on July 22, 2001, when Cluster was located in the postmidnight magnetotail. The flow burst was associated with a clear dipolarization ahead of the high‐speed part of the predominantly Earthward directed flow. Based on the analysis of the four spacecraft data, we found that a ∼2000 km thick dipolarization front moves Earthward and dawnward with a speed of ∼77 km/s. The plasma before this front is deflected, consistent with the plasma ahead of a localized plasma bubble centered at midnight side being pushed aside by the moving obstacle. The main body of the high‐speed flow is directed mainly parallel to the dipolarization front. These observations indicate that the evolution of the dipolarization front across the tail is directly coupled with the fast flow.

[1] Spatial gradients of high-speed flows in the midtail plasma sheet are determined using multipoint observations from the Cluster spacecraft along the ''dawn-dusk'' direction (perpendicular to the main flow and in the plane of the tail current sheet) and along the north-south direction. If we take the average or median of the spatial gradients and assume that the flow channel has a linear gradient, these values suggest that the full width of the flow channel is 2-3 R E in the ''dawn-dusk'' direction and 1.5-2 R E in the north-south direction. The velocity gradient at the duskward edge of a flow tends to be sharper than that at the dawnward edge, possibly reflecting an asymmetry in the magnetosphere-ionosphere coupling process associated with the flow.

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 RE 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 ∣Bx∣ ∼ 0.5 BL, with a pronounced current density minimum and a plasma density plateau between these peaks.

[1] During the interval 0947 -0951 UT on 1 October 2001, when Cluster was located at X GSM = À16.4 R E near Z GSM = 0 in the pre-midnight magnetotail, the Cluster barycenter crosses the neutral sheet four times. High speed proton flow, with reversal from tailward to Earthward, was detected during the crossings. Using a linear gradient/curl estimator technique we estimate current density and magnetic field curvature within the crossings. These observations exhibit the tailward passage of an X-line over the Cluster tetrahedron. These current sheet has a bifurcated structure in the regions of tailward and earthward flows and a flat and/or slightly bifurcated thin current sheet in between. A distinct quadrupolar Hall magnetic field component was observed.

Abstract. Using four-point magnetic field measurements by the Cluster spacecraft, we statistically analyze the magnetic field and electric current configurations during rapid crossings of the current sheet observed in July-October 2001 at geocentric distances of 19 R E . The database includes 78 crossings, specially selected to apply multi-point data analysis techniques to calculate vector derivatives. Observed bipolar variations of j z , often with |j z |>j y , indicate that the electric currents follow kinks of the current sheet. The current density varies between 5-25 nA/m 2 . The half-thickness of the current sheet during flapping varies over a wide range, from 1 to 20 ion thermal gyroradii (L cp , calculated from average temperature and lobe magnetic field for each crossing). We found no relationship between the tilt angle of the current sheet normal and the half-thickness. In 68 cases the magnetic field curvature vector has a positive (earthward) X-component. Ten cases with a negative (tailward) curvature, associated with reconnection, were detected within 0