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It is well known that nonzero IMF By conditions\udlead to a twisted magnetotail configuration. The plasma sheet\udis rotated around its axis and tail magnetic field lines are\udtwisted, which causes an azimuthal displacement of their\udionospheric footprints. According to the untwisting hypothesis,\udthe untwisting of twisted field lines is suggested to influence\udthe azimuthal direction of convective fast flows in the\udnightside geospace. However, there is a lack of in situ magnetospheric\udobservations, which show actual signatures of\udthe possible untwisting process. In this paper, we report detailed\udCluster observations of an azimuthal flow shear across\udthe neutral sheet associated with an earthward fast flow on\ud5 September 2001. The observations show a flow shear velocity\udpattern with a V⊥y sign change, near the neutral sheet (Bx ∼ 0) within a fast flow during the neutral sheet flapping\udmotion over the spacecraft. Firstly, this implies that convective\udfast flows may not generally be unidirectional across the\udneutral sheet, but may have a more complex structure. Secondly,\udin this event tail By and the flow shear are as expected by the untwisting hypothesis. The analysis of the flow shear\udreveals a linear dependence between the Bx and V⊥y close to\udthe neutral sheet and suggests that Cluster crossed the neutral\udsheet in the dawnward part of the fast flow channel. The magnetospheric\udobservations are supported by the semi-empirical T96 and TF04 models. Furthermore, the ionospheric SuperDARN\udconvection maps support the satellite observations\udproposing that the azimuthal component of the magnetospheric\udflows is enforced by a magnetic field untwisting. In\udsummary, the observations give strong supportive evidence to the tail untwisting hypothesis. However, the T96 ionospheric\udmapping demonstrates the limitations of the model in mapping\udfrom a twisted tail.The work by TP and MH was supported by the Swedish\udNational Space Board project 78/11ABPeer-reviewedPublisher Versio

Azimuthal velocity shear within an Earthward fast flow – further evidence for magnetotail untwisting?