资源环境科技发展态势分析平台

Flux Transfer Events (FTEs) are transient phenomena produced by magnetic reconnection at the dayside magnetopause typically under southward interplanetrary magnetic field (IMF) conditions. They are usually thought of as magnetic flux ropes with helical structures forming through patchy, unsteady or multiple X‐line reconnection. While the IMF often has a non‐zero B_Y component, its impacts on the FTE flux rope helicity remain unknown. We survey Magnetospheric Multiscale (MMS) observations of FTE flux ropes during years 2015 ‐ 2017 and investigate the solar wind conditions prior to the events. By fitting a force‐free flux rope model, we select 84 events with good fits and obtain the helicity sign (i.e., handedness) of the flux ropes. We find that positive (negative) helicity flux ropes are mainly preceded by positive (negative) B_Y component. This finding is compatible with flux ropes formed through a multiple X‐lines mechanism.