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. We present Cluster spacecraft observations from 12 October 2006 ofconvective plasma flows in the Earth's magnetotail. Earthward flow burstswith a dawnward v⊥y component, observed by Cluster 1 (C1), areinconsistent with the duskward flow that might be expected at thepre-midnight location of the spacecraft. Previous observations havesuggested that the dusk–dawn sense of the flow can be governed by theinterplanetary magnetic field (IMF) By conditions, with the related“untwisting hypothesis” of magnetotail dynamics commonly invoked to explain this dependence, in terms of a large-scale magnetospheric asymmetry. In the current study, observations of the upstream solar wind conditions from OMNI, magnetic field observations by Cluster and ionospheric convection data using SuperDARN indicate a large-scale magnetospheric morphology consistent with positive IMF By penetration into the magnetotail. At the pre-midnight location of Cluster, however, the dawnward flow observed below the neutral sheet by C1 could only be explained by the untwisting hypothesis in a negative IMF By scenario. The Cluster magnetic field data also reveal a flapping of the magnetotail current sheet, a phenomenon known to influence dusk–dawn flow. Results from the curlometer analysis technique suggest that the dusk–dawn sense of the J×B force was consistent with localised kinks in the magnetic field and the flapping associated with the transient perturbations to the dusk–dawn flow observed by C1. We therefore suggest that the flapping overcame the dusk–dawn sense of the large-scale convection which we would expect to have been net duskward in this case. We conclude that invocation of the untwisting hypothesis may be inappropriate when interpreting intervals of dynamic magnetotail behaviour such as during current sheet flapping, particularly at locations where magnetotail flaring becomes dominant.

Dynamics of variable dusk–dawn flow associated with magnetotail current sheet flapping