Reconstruction of a bipolar magnetic signature in an earthward jet in the tail: Flux rope or 3D guide-field reconnection?

摘要

Southward-then-northward magnetic perturbations are often seen in the tail plasma sheet, along with earthward jets, but the generation mechanism of such bipolar B z (magnetic flux rope created through multiple X-line reconnection, transient reconnection, or else) has been controversial. At ～2313 UT on 13 August 2002, Cluster encountered a bipolar B z at the leading edge of an earthward jet, with one of the four spacecraft in the middle of the current sheet. Application to this bipolar signature of Grad-Shafranov (GS) reconstruction, the technique for recovery of two-dimensional (2D) magnetohydrostatic structures, suggests that a flux rope with diameter of ～2 R E was embedded in the jet. To investigate the validity of the GS results, the technique is applied to synthetic data from a three-dimensional (3D) MHD simulation, in which a bipolar B z can be produced through localized (3D) reconnection in the presence of guide field B y (Shirataka et al., 2006) without invoking multiple X-lines. A flux rope-type structure, which does not in fact exist in the simulation, is reconstructed but with a shape elongated in the jet direction. Unambiguous identification of a mechanism that leads to an observed bipolar B z thus seems difficult based on the topological property in the GS maps. We however infer that a flux rope was responsible for the bipolar pulse in this particular Cluster event, because the recovered magnetic structure is roughly circular, suggesting a relaxed and minimum energy state. Our results also indicate that one has to be cautious about interpretation of some (e.g., force-free, or magnetohydrostatic) model-based results.