IMF control of the azimuthal direction of earthward magnetotail fast flows?

Cluster magnetotail data together with ACE solar wind data from 2001-2009 are used to investigate the dependence of the azimuthal flow direction of earthward magnetotail fast flows on the interplanetary magnetic field (IMF). We find indication that fast flows have favorable azimuthal directions that are dependent on the IMF. Our results suggest that for positive IMF By the favorable azimuthal direction of the fast flows is dawnward in the northern plasma sheet and duskward in the southern plasma sheet. For negative IMF By an opposite situation takes place, the favorable azimuthal flow directions are then duskward and dawnward in the northern and southern plasma sheet, respectively. The results are in agreement with the idea that untwisting of reconnected magnetic field lines directs the fast flows in the magnetotail, the field line twist itself being controlled by the IMF.