A Statistical Study of the Force Balance and Structure in the Flux Ropes in Mercury's Magnetotail

This study presents a statistical investigation of the force balance and structures in the flux ropes in Mercury's magnetotail plasma sheet by using the measurements of MErcury Surface, Space ENviroment, GEochemistry, and Ranging (MESSENGER). One hundred sixty‐eight flux ropes were identified from the 14 hot seasons of MESSENGER from 11 March 2011 to 30 April 2015, and 143 of them show clear magnetic field enhancements with the core field being ≥20% higher than the background magnetic field. The investigation on the force balance of these 143 flux ropes shows that magnetic pressure gradient force cannot be solely balanced by magnetic tension force, implying that thermal plasma pressure gradient force cannot be neglected in the flux ropes. We employ a non‐force‐free model considering the contribution of thermal pressure to resolve the physical properties of flux ropes in Mercury's magnetotail. Twenty‐eight flux ropes are obtained through the fitting to the non‐force‐free model. The flux ropes are found to be consistent with the flattened structures, in which the mean semimajor is ∼851 km and semiminor is ∼333 km, both are several times the local proton inertial length. The average core field is estimated to be ∼57.5 nT, and flux content is ∼0.019 MWb, much larger than the previous results obtained from force‐free flux rope model. The importance of thermal pressure gradient in the force balance of the flux ropes and the flattened structure indicates that the flux ropes in Mercury's magnetotail plasma sheet are mostly in early stage of the evolution, and still contain enough plasma to affect their magnetic structures. Key Points Thermal pressure gradient is significant for the flux ropes in Mercury's magnetotail Non‐force‐free modeling reveals the flatten structure and much higher magnetic flux of the flux ropes different from the previous studies Flux ropes in this study should be in their early stage of evolution and could be strongly affected by thermal pressure