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Flux transfer events (FTEs) are transient phenomena frequently observed at the Earth's magnetopause. Their usual interpretation is a flux rope moving away from the reconnection region. However, the Magnetospheric Multiscale Mission revealed that magnetic reconnection sometimes occurs inside these structures, questioning their flux rope configuration. Here we investigate 229 FTE‐type structures and find reconnection signatures inside 19% of them. We analyze their large‐scale magnetic topology using electron heat flux and find that it is significantly different across the FTE reconnecting current sheets, demonstrating that they are constituted of two magnetically disconnected structures. We also find that the interplanetary magnetic field (IMF) associated with reconnecting FTEs presents a strong By component. We discuss several formation mechanisms to explain these observations. In particular, the maximum magnetic shear model predicts that for large IMF By, two spatially distinct X lines coexist at the magnetopause. They can generate separate magnetic flux tubes that may become interlaced. Plain Language Summary: The solar wind and the Earth's magnetosphere are two gigantic magnetic structures that collide constantly over our heads, in the near‐space environment. At the boundary of their interaction (the magnetopause), the fundamental process of magnetic reconnection can occur. It is there that dynamic magnetic structures called "flux transfer events" are formed. They travel fast along the magnetopause and transport a lot of energy, from the solar wind into the magnetosphere. These structures are yet not well understood, as underlined by the recent observations made by the Magnetospheric Multiscale Mission (MMS), launched in 2015 by National Aeronautics and Space Administration. The four‐spacecraft mission, specifically designed to study the physics happening at the magnetopause, is capable of measuring right into these magnetic structures, collecting data on their particles and magnetic field properties. When analyzing MMS data, we found that 19% of the flux transfer events were not constituted of one, but two structures with very different properties. These dual magnetic structures tend to appear when the solar wind's magnetic field is oriented mainly toward the east or the west. From these observations and based on existing models of the magnetopause, we propose a scenario that allows such dual structures to form as interlaced magnetic tubes. Key Points: Nineteen percent of FTE‐type structures observed by MMS during Phases 1A and 1B present signatures of magnetic reconnection in their coreThey seem to be formed by two magnetically disconnected interlaced flux tubes and are typically observed for large IMF BySeveral formation models are discussed, including a bifurcated X line scenario that results from the maximum shear angle model [ABSTRACT FROM AUTHOR]

Flux transfer events (FTEs) are transient phenomena produced by magnetic reconnection at the dayside magnetopause typically under southward interplanetary 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 BY 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) BY component. This finding is compatible with flux ropes formed through a multiple X‐lines mechanism. Plain Language Summary: The Earth's near‐space environment is very dynamic with transient phenomena triggered by interaction between the solar wind and the Earth's magnetopause. The solar wind carries along an interplanetary magnetic field (IMF) whose orientation determines the dynamics of the interaction. When the IMF is southward, magnetic reconnection can be triggered at the Earth's magnetopause on the dayside. A flux transfer event (FTE) is a transient portal that allows the bursty transfer of solar wind into the Earth's magnetosphere. An FTE is envisaged as a twisted magnetic field structure with helical field that looks like a rope. We study the relationship between the IMF orientations and the twist direction of an ensemble of FTEs observed by NASA's Magnetospheric Multiscale mission, by modeling FTEs as magnetic flux ropes. We found that the flux rope twist direction is controlled by the IMF orientation, such that the rope is twisted in the left‐handed or right‐handed sense depending on the east‐west component of the IMF. This result supports the formation of FTEs by a multiple reconnection mechanism. Key Points: The helicity sign of 84 flux transfer events (FTEs) is studied using force‐free flux rope model fittingRight‐handed (left‐handed) FTE flux ropes are mostly preceded by positive (negative) interplanetary magnetic field (IMF) BYThis IMF BY control of the helicity sign is compatible with a multiple X‐line formation mechanism [ABSTRACT FROM AUTHOR]