Electric current and magnetic field geometry in flapping magnetotail current sheets

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 R E . The database includes 78 crossings, specially selected to apply multi-point data analysis techniques to calculate vector derivatives. Observed bipolar variations of j z , often with |j z |>j y , indicate that the electric currents follow kinks of the current sheet. The current density varies between 5-25 nA/m 2 . The half-thickness of the current sheet during flapping varies over a wide range, from 1 to 20 ion thermal gyroradii (L cp , 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<Y GSM <7 R E . The minimum curvature radii vary mainly between 1 and 10 L cp , 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 (|B x |<5 nT) 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.

Double Star/Cluster observation of neutral sheet oscillations on 5 August 2004

“…A schematic view of this wave is shown in Fig. 4 based on this study and earlier observations (Zhang et al, 2002;Sergeev et al, 2004;Runov et al, 2005b). In order to estimate the amplitude of this wave, we performed a detailed analysis of each Cluster neutral sheet crossing by calculating the Cluster tetrahedron barycenter transverse distance from the neutral sheet (Runov et al, 2005a).…”

“…The magnetotail neutral sheet is anchored to the geomagnetic dipole at a hinging distance of 11 Re (Dandouras, 1988). As mentioned in the introduction, the neutral sheet kink-like wave along the Y-axis has been observed at tailward distances of ∼14 to 30 Re (Lui et al, 1978;Runov et al, 2005b). The present study shows that this wave can be observed as close to the Earth as 11 Re along the X direction, in the neighbourhood of the magnetotail hinge point.…”

“…In order to estimate the amplitude of this wave, we performed a detailed analysis of each Cluster neutral sheet crossing by calculating the Cluster tetrahedron barycenter transverse distance from the neutral sheet (Runov et al, 2005a). Except for the first crossing, all crossings indicate a wave amplitude between 4000 and 8000 km which is about the same as the statistical result of 1 Re (Runov et al, 2005b). Further we infer that the wavelength is about 4 Re by taking a wave propagation velocity of ∼20 km/s and a wave period of ∼20 min estimated from Table 1 and Table 2.…”

“…Neutral sheet kink-like waves along the Y-axis have been observed at tailward distances of ∼15 to 30 Re by IMP 5 (Lui et al, 1978) and around ∼14 to 19 Re by Cluster (Runov et al, 2005b). In addition, Volwerk et al (2003) found kinkmode waves along the X-axis.…”

“…Further statistical studies revealed that this dawn-dusk wave is kink-like and has a preCorrespondence to: T. L. Zhang (tielong.zhang@oeaw.ac.at) ferred propagation direction from the tail center to flank (Sergeev et al, 2004;Runov et al, 2005b). Thus the source of this kink-like wave is localized in the central part of the magnetotail.…”