Statistical Relationship Between the Solar Wind Conditions and the Helicity Sign of Flux Transfer Events at the Earths Magnetopause: Cluster and MMS Observations

Flux Transfer Events (FTEs) are transient phenomena generated by magnetic reconnection at the dayside magnetopause. They are described as magnetic flux ropes with helicoidal structures, which are believed to form due to single, unsteady or multiple X-line reconnection. A recent study shows that the flux rope twist direction, described by the helicity sign, is mostly associated with the direction of the Interplanetary Magnetic Field (IMF). We survey a list of events observed by Cluster and MMS. The events are described with a force-free flux rope model, from which we investigated their helicity sign. We investigate solar wind conditions preceding the events. We find that some of the events are compatible with multiple X-line formation mechanism; however, a significant group is not consistent with this mechanism. To explain this unexpected population, we survey the shear angle at FTE locations using the Maximum Magnetic Shear model (Trattner et al. [2007]). We find that most events of this group locate at large shear angles (i.e., where anti-parallel reconnection is expected). We will present our analyses and discuss how FTE flux ropes gain their helicity sign based on multiple X-line reconnection and possibly alternative mechanisms.