Abstract
Abstract. Thirty rapid crossings of the magnetotail current sheet by the Cluster spacecraft during July-October 2001 at a geocentric distance of 19 RE are examined in detail to address the structure of the current sheet. We use four-point magnetic field measurements to estimate electric current density; the current sheet spatial scale is estimated by integration of the translation velocity calculated from the magnetic field temporal and spatial derivatives. The local normal-related coordinate system for each case is defined by the combining Minimum Variance Analysis (MVA) and the curlometer technique. Numerical parameters characterizing the plasma sheet conditions for these crossings are provided to facilitate future comparisons with theoretical models. Three types of current sheet distributions are distinguished: center-peaked (type I), bifurcated (type II) and asymmetric (type III) sheets. Comparison to plasma parameter distributions show that practically all cases display non-Harris-type behavior, i.e. interior current peaks are embedded into a thicker plasma sheet. The asymmetric sheets with an off-equatorial current density peak most likely have a transient nature. The ion contribution to the electric current rarely agrees with the current computed using the curlometer technique, indicating that either the electron contribution to the current is strong and variable, or the current density is spatially or temporally structured.
Highlights
Current sheet structure is an important property of plasma boundaries, which determines their stability against perturbations and explosive disruptions
The basic criteria for selection of the rapid current sheet crossings are discussed in our previous paper (Runov et al, 2005a). They are (i) the change in the magnetic field X-component at the Cluster tetrahedron barycenter larger than 15 nT, during a time less than 5-min, with a change in the Bx sign, indicating the neutral sheet crossing; (ii) the current sheet is stable during the crossing; and (iii) the ratio of the magnetic field divergence and curl is less than 0.25 for more than 60% of samples during the crossing
Because the minima of the magnetic field curvature radius were found within the neutral sheet in all cases, the above-written κ-parameter definition was used for bifurcated sheets, too
Summary
Flapping motion of the magnetotail current sheet manifests itself as both large-amplitude (a few tens of nT) and short duration (tens of seconds to several minutes), often repeating variations of the magnetic field main component, observed by spacecraft in the plasma sheet. Using another method, fitting ISEE 1/2 magnetic field data to the Harris function, Sanny et al (1994) inferred that the current sheet thickness at X∼–13 RE varied from several RE at the beginning of the substorm growth phase to h∼0.1 RE just after expansion onset They suggested a multi-point calculation approach, based on calculations of the convective derivative, which allows one to reconstruct an effective vertical scale of the current sheet during its crossing by a group of spacecraft. Analyzing the Bx-occurrence frequency distribution during multiple current sheet crossings (expected to be inversely proportional to dBx/dz gradients in the case of vertical flapping motions), measured by the Geotail spacecraft at ∼100 RE in the tail, Hoshino et al (1996) found distributions consistent with a double-peaked (bifurcated) current density profile Because such structures were observed during fast ion flows, Hoshino et al (1996) attributed them to slow shock structures downstream of a reconnection site. The 1-s averaged magnetic field data from the Cluster Flux Gate Magnetometer (FGM, Balogh et al, 2001) and 2-spin (normal mode) or 1-spin (burst mode) averaged data from the Cluster Ion Spectrometry experiment (CIS, Reme et al, 2001) are used in the analysis presented in this paper
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call