Double‐peaked core field of flux ropes during magnetic reconnection

A flux rope event observed in the magnetotail exhibits a double‐peaked core field feature. The generation of such double‐peaked feature within the flux rope is explored with Hall‐MHD simulations and theoretical analysis based on multiple X line reconnection. Simulations with a guide field produce flux ropes bounded by two active X lines in the thin current sheet. The guide field, combined with Hall‐generated field, leads to a donut‐shaped core field (having a double‐peaked profile) near the magnetic separatrix. Subsequently, it rotates into the central region of the flux rope, which tends to be the force‐free configuration. The analysis shows that there are three major factors affecting the evolution of the core field, including the guide field, convective, and Hall terms originating from the generalized Ohm's law. The convective term can become stronger near the central region of flux rope, and the Hall term dominates the region next to the separatrix during the early stages of the flux rope evolution. It implies that several different factors contribute to the generation of the double‐peaked core field. The results may help explain a variety of core fields available in magnetotail flux ropes. Key Points Cluster observation and Hall‐MHD simulation used to investigate the flux rope structure Explore the generation mechanism of the core field within the flux rope Hall effect plays a key role in causing the double‐peaked core field