Statistics of intense dawn-dusk currents in the Earth's magnetotail

We consider Cluster observations of events with an intense current density (> 5 nA/m(2)) in the magnetotail current sheet. We use measurements by Cluster mission in the central magnetotail (X < - 16 R-E, vertical bar Y vertical bar < 10 R-E, and vertical bar B-x vertical bar < 10 nT) in the 2003 season when the spacecraft separation was about similar to 300 km. For this season, when Cluster can probe very small scale currents, we collect the statistics of observations of strong current densities j(y) (in GSM coordinate system) and compare curlometer data with plasma parameters. The most intense currents are observed under disturbed conditions (plasma flow v(x) > 300 km/s). We introduce the parameter v(D)/v(Ti) (where v(D) = j(y)/en(e), n(e) is an electron density, and v(Ti) is a proton thermal velocity) and show that cases with v(D)/v(Ti) similar to 1 correspond to the most intense currents observed in the vicinity of the reconnection regions. The comparison of electron and proton velocities demonstrates that electron often carry almost the entire current measured by a curlometer technique. The strong electron temperature anisotropy T-e parallel to/T-e perpendicular to > 1.2 corresponds to large magnitudes of B-y component of the magnetic field. We conclude that intense current sheets are often characterized by significant (more than 30%) contribution of electron curvature currents to the cross-tail current. The comparison of observations and models shows that the electron anisotropy level is likely controlled by competition of two processes: the electron scattering on fluctuations generated by fire-hose instability and the acceleration in sheared magnetic field configurations. We also suggest that current sheets embedded into the strong plasma flows (v(x)/v(Ti) > 0.1) can be balanced by ion flow anisotropy.