[1] We report THEMIS observations of a dipolarization front, a sharp, large-amplitude increase in the Z-component of the magnetic field. The front was detected in the central plasma sheet sequentially at X = À20.1 R E (THEMIS P1 probe), at X = À16.7 R E (P2), and at X = À11.0 R E (P3/P4 pair), suggesting its earthward propagation as a coherent structure over a distance more than 10 R E at a velocity of 300 km/s. The front thickness was found to be as small as the ion inertial length. Comparison with simulations allows us to interpret the front as the leading edge of a plasma fast flow formed by a burst of magnetic reconnection in the midtail.

[1] We discuss results of a superposed epoch analysis of dipolarization fronts, rapid (dt < 30 s), high-amplitude (dB z > 10 nT) increases in the northward magnetic field component, observed during six Time History of Events and Macroscale Interactions during Substorms (THEMIS) conjunction events. All six fronts propagated earthward; time delays at multiple probes were used to determine their propagation velocity. We define typical magnetic and electric field and plasma parameter variations during dipolarization front crossings and estimate their characteristic gradient scales. The study reveals (1) a rapid 50% decrease in plasma density and ion pressure, (2) a factor of 2-3 increase in high-energy (30-200 keV) electron flux and electron temperature, and (3) transient enhancements of ∼5 mV/m in duskward and earthward electric field components. Gradient scales of magnetic field, plasma density, and particle flux were found to be comparable to the ion thermal gyroradius. Current densities associated with the B z increase are, on average, 20 nA/m 2 , 5-7 times larger than the current density in the cross-tail current sheet. Because j · E > 0, the dipolarization fronts are kinetic-scale dissipative regions with Joule heating rates of 10% of the total bursty bulk flow energy.

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.

Earth's magnetotail contains magnetic energy derived from the kinetic energy of the solar wind. Conversion of that energy back to particle energy ultimately powers Earth's auroras, heats the magnetospheric plasma, and energizes the Van Allen radiation belts. Where and how such electromagnetic energy conversion occurs has been unclear. Using a conjunction between eight spacecraft, we show that this conversion takes place within fronts of recently reconnected magnetic flux, predominantly at 1- to 10-electron inertial length scale, intense electrical current sheets (tens to hundreds of nanoamperes per square meter). Launched continually during intervals of geomagnetic activity, these reconnection outflow flux fronts convert ~10 to 100 gigawatts per square Earth radius of power, consistent with local magnetic flux transport, and a few times 10(15) joules of magnetic energy, consistent with global magnetotail flux reduction.

Abstract. Thirty rapid crossings of the magnetotail current sheet by the Cluster spacecraft during July-October 2001 at a geocentric distance of 19 R E are examined in detail to address the structure of the current sheet. We use four-point magnetic field measurements to estimate electric current density; the current sheet spatial scale is estimated by integration of the translation velocity calculated from the magnetic field temporal and spatial derivatives. The local normalrelated coordinate system for each case is defined by the combining Minimum Variance Analysis (MVA) and the curlometer technique. Numerical parameters characterizing the plasma sheet conditions for these crossings are provided to facilitate future comparisons with theoretical models. Three types of current sheet distributions are distinguished: centerpeaked (type I), bifurcated (type II) and asymmetric (type III) sheets. Comparison to plasma parameter distributions show that practically all cases display non-Harris-type behavior, i.e. interior current peaks are embedded into a thicker plasma sheet. The asymmetric sheets with an off-equatorial current density peak most likely have a transient nature. The ion contribution to the electric current rarely agrees with the current computed using the curlometer technique, indicating that either the electron contribution to the current is strong and variable, or the current density is spatially or temporally structured.

[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.