The Role of Magnetic Flux Rope in Ion Acceleration: MHD Simulations and Test-particle Tracing

Magnetic flux ropes (MFRs), playing a crucial role in particle energization and energy transport in the solar–terrestrial space, are helical structures produced by magnetic reconnection. It has been both theoretically predicted and observationally confirmed that MFRs and associated processes are inherently three-dimensional in space. Although such structures have been widely suggested as a favorable place for electron acceleration, whether large-scale MFRs can lead to ion acceleration has been rarely investigated. In this study, an MHD model is used to examine the evolution of large-scale MFRs in the magnetotail, and a test-particle simulation is further employed to study the associated ion energization. Results show that magnetic reconnections take place at multiple X-lines in the magnetotail current sheet, generating a twisted MFR with a scale of about 10 R e in azimuth. It propagates earthward following the tail reconnection but its east and west wings are significantly distorted azimuthally. Test-particle tracing reveals that ions (0.1–100 keV) can be trapped within the rope while being effectively accelerated. The rope therefore brings in energetic plasma sources into the inner magnetosphere as it transports earthward. These results demonstrate that the MFR is an important source carrier for the ring-current formation in the inner magnetosphere.