Ion Velocity Distributions in a Small-Scale Magnetic Flux Rope: A Case Study

Magnetic flux rope is of a helical magnetic field configuration generated by magnetic reconnection. A magnetic flux rope is characterized by a bipolar magnetic field component normal to a boundary layer and an increase of magnetic field strength at the reversal of the bipolar field. Ion behaviors have rarely been investigated in a small-scale magnetic flux rope due to a lack of high-time-resolution ion plasma measurements. In this paper, a case study is presented using Magnetospheric Multiscale observations of a small-scale flux rope during a magnetopause crossing from magnetosphere to magnetosheath. The diameter of the flux rope was ~ 236 km, corresponding to ~ 5.4 ion inertial lengths. In the negative normal field region, crescent-shaped ion velocity distributions were formed in a plane perpendicular to magnetic field, but their crescent distributions were not similar. By contrast, different-shaped velocity distributions were found in the positive normal field region. Moreover, the velocity distribution map at the flux rope center was different than others. Although the ion motions were different within the flux rope, they all had a positive E \(\times\) B drift.