Statistical survey on the magnetic structure in magnetotail current sheets

On the basis of the multipoint magnetic observations of Cluster in the region 15-19 R-E downtail, the magnetic field structure in magnetotail current sheet (CS) center is statistically surveyed. It is found that the B-y component (in GSM coordinates) is distributed mainly within vertical bar B-y vertical bar < 5nT, while the B-z component is mostly positive and distributes mainly within 1 similar to 10 nT. The plane of the magnetic field lines (MFLs) is mostly vertical to the equatorial plane, with the radius of curvature (Rc) of the MFLs being directed earthward and the binormal (perpendicular to the curvature and magnetic field direction) being directed azimuthally westward. The curvature radius of MFLs reaches a minimum, R-c,R-min, at the CS center and is larger than the corresponding local half thickness of the neutral sheet, h. Statistically, it is found that the overall surface of the CS, with the normal pointing basically along the south-north direction, can be approximated to be a plane parallel to equatorial plane, although the local CS may be flapping and is frequently tilted to the equatorial plane. The tilted CS (normal inclined to the equatorial plane) is apt to be observed near both flanks and is mainly associated with the slippage of magnetic flux tubes. It is statistically verified that the minimum curvature radius, R-c,R-min, half thickness of neutral sheet, h, and the slipping angle of MFLs, delta, in the CS satisfies h = R-c,R-min cos delta. The current density, with a mean strength of 4-8 nA/m(2), basically flows azimuthally and tangentially to the surface of the CS, from dawn side to the dusk side. There is an obvious dawn-dusk asymmetry of CS, however. For magnetic local times (MLT) similar to 21:00-similar to 01:00, the CS is relatively thinner; the minimum curvature radius of MFLs, R-c,R-min (0.6-1 R-E) and the half-thickness of neutral sheet, h (0.2-0.4 R-E), are relatively smaller, and B-z (3-5 nT) and the minimum magnetic field, B-min (5-7 nT), are weaker. It is also found that negative B-z has a higher probability of occurrence and the cross-tail current density j(Y) is dominant (2-4 nA/m(2)) in comparison to those values near both flanks. This implies that magnetic activity, e. g., magnetic reconnection and current disruption, could be triggered more frequently in CS with similar to 21:00-similar to 01:00 MLT. Accordingly, if mapped to the region in the auroral ionosphere, it is expected that substorm onset would be optically observed with higher probability for similar to 21:00-similar to 01:00 MLT, which is well in agreement with statistical observations of auroral substorm onset.