Abstract
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 RE. The database includes 78 crossings, specially selected to apply multi-point data analysis techniques to calculate vector derivatives. Observed bipolar variations of jz, often with | jz |>jy, indicate that the electric currents follow kinks of the current sheet. The current density varies between 5-25nA/m2. The half-thickness of the current sheet during flapping varies over a wide range, from 1 to 20 ion thermal gyroradii (Lcp), 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<YGSM<7 RE. The minimum curvature radii vary mainly between 1 and 10 Lcp, and the adiabaticity parameter κ≤1 for 73% of the events. The electric current density during flapping is often off-central, i.e. the main current density is shifted from the neutral sheet (| Bx |<5nT) to the Northern or Southern Hemisphere. This is most likely a temporal effect related to the flapping. The analysis shows that the flapping motion of the current sheet is associated with kink-like waves on the sheet surface. The kink fronts, tilted in the Y-Z plane, moved toward dawn in the morning half and toward dusk in the evening half of the magnetotail.
Highlights
Speiser and Ness (1967) noticed that the magnetotail current sheet frequently moves in the north-south direction
In this paper we provide a statistical survey of the current sheet geometry, magnetic field gradient, curvature, and current density distribution during rapid crossings of the sheet during the 2001 Cluster tail season
The Z-component of the electric current density j=μ−0 1∇×B calculated from four-point magnetic field measurements was the largest component during both crossings, and showed a bipolar variation: jz>0 during the first crossing and jz
Summary
Speiser and Ness (1967) noticed that the magnetotail current sheet frequently moves in the north-south direction This motion, which has been referred to as flapping (Lui et al, 1978), provides the possibility to probe the structure of the current sheet during multiple crossings (Sergeev et al, 1993) and deduce an electric current density within the sheet (Sergeev et al, 1998). Case studies of the magnetotail current sheet with the Cluster four spacecraft constellation show that short-term (∼30 s − several minutes), large-scale variations (δBx∼15– 30 nT) of the magnetic field, which are typical manifestations of flapping, are associated mainly with wave-like structures on the current sheet surface, propagating in the cross tail direction (Zhang et al, 2002; Runov et al, 2003b; Sergeev et al, 2003). In this paper we provide a statistical survey of the current sheet geometry, magnetic field gradient, curvature, and current density distribution during rapid crossings of the sheet during the 2001 Cluster tail season
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