Local Structure of the Magnetotail Current Sheet: Cluster Observations

Current sheet structure is an important property of plasma boundaries, which determines their stability. Because of difficulty to study current sheets in space with single spacecraft measurements, interpretations of data as well as theoretical model was mostly based on a simplest 1D solution, known as Harris sheet. With four-point observations by the Cluster spacecraft, we are able at the first time unambiguously reconstruct local structure of current sheets in the Earth magnetosphere. In this paper we summarize the Cluster probes of the magnetotail current sheet. The Cluster tetrahedron scale varied from 200 to 2000 km, which is comparable with the ion thermal gyroradius and with ion inertial scale in the magnetotail current sheet. Using a set of 1 - 5 min-long crossings of the current sheet, often during so-called flapping events, and applying the linear gradient estimator technique, we study spatial profiles of the current density, ion density, temperature, and velocity in the current sheet. It was found that (i) in the majority of cases studied, the current sheet is embedded into a thicker plasma sheet; (ii) most of the current density distributions differ from the Harris function; (iii) in about 55% of the cases the current sheet has a single peak of the current density at B_x~0 and a typical scale of 5 external ion gyroradii Lcp (calculated using the lobe magnetic field), in about 10% of the cases the current sheet was bifurcated with two quasi-symmetric thin (3 - 5 Lcp) peaks of the current density, and in about 35% of the crossings the current density profiles are asymmetric with an off-equatorial maximum. Case studies show that the asymmetric off-center current sheets are transient, with a life-time of several tens of seconds. Scales of these structures vary between 3 - 7 Lcp. Alternatively, using a different, somewhat ``orthogonal'' data set that avoids periods of flapping, large tilt of the current sheet and fast flows, and a different method that compares the inner (at |B_x|<0.3 B_L, where B_L is the lobe magnetic field) and outer (at 0.3<|B_x|<0.7 B_L) central plasma sheet, off-center peaks were found in 17% of all observations. For the center-peaked current sheets, very intense current frequently appears in the center, indicating an embedded structure.