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

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.