Evidence of guide field magnetic reconnection in flapping current sheets

Based on current sheet flapping motion on 27 August 2018 in the dusk flank magnetotail, as recorded by instruments aboard Magnetospheric Multiscale (MMS) spacecraft, we present the first study of guide field reconnection observed in the flux rope embedded in kink-like flapping current sheets near the dusk-side flank of the magnetotail. Unlike more common magnetotail reconnections, which are symmetric, these asymmetric small-scale (λ_i ~ 650 km) reconnections were found in the highly twisted current sheet when the direction normal to the sheet changes from the Z direction into the Y direction. The unique feature of this unusual reconnection is that the reconnection jets are along the Z direction — different from outflow in the X direction, which is the more usual situation. This vertical reconnection jet is parallel or antiparallel to the up-and-down motion of the tail’s current sheet. The normalized reconnection rate R is estimated to be ~ 0.1. Our results indicate that such asymmetric reconnections can significantly enlarge current sheet flapping, with large oscillation amplitudes. This letter presents direct evidence of guide field reconnection in a highly twisted current sheet, characterized by enlarged current sheet flapping as a consequence of the reconnection outflow.