Statistical relationship between IMF conditions and the helicity of Flux Transfer Event flux ropes

Abstract:

Flux Transfer Events (FTEs) are transient phenomena produced by magnetic reconnection at the dayside magnetopause. They are usually thought of as magnetic flux ropes with helical structures forming through unsteady or multiple X-line reconnection. They are mainly observed under southward Interplanetary Magnetic Field (IMF) conditions. While the IMF often have a duskward/dawnward (B_Y) component, impacts of the B_Y component on the FTE flux rope helicity remain unclear. We survey Magnetospheric Multiscale (MMS) observations of 186 FTE flux ropes in 2015 – 2017 and investigate the solar wind conditions prior to the events. By fitting a force-free flux rope model, we select events with fairly good fits and extract model parameters to statistically analyze the characteristics of the flux ropes. We find that there is a relationship between the IMF B_Y orientation and the helicity of the flux ropes. These results are consistent with flux ropes formed through a multiple X-lines mechanism.