Internal structures of the ion-scale flux rope associated with dayside magnetopause reconnection

Abstract

Ion-scale magnetic flux ropes have been observed in the reconnection diffusion region. In this paper, we show a typical ion-scale flux rope observed by the Magnetospheric Multiscale spacecraft at the subsolar magnetopause. The most intriguing feature of this flux rope is that the rapid variations of electron density are correlated with the changes of electron temperature and pitch angle distribution inside the flux rope. In the region of low electron density, the perpendicular temperature is higher than parallel temperature. Furthermore, the electrons within higher energies (>70 eV) are mainly trapped near pitch angle $\sim 90^{\circ}$, but electrons with lower energies (<70 eV) are missing. However, in the region of high electron density, perpendicular temperature is lower than the parallel temperature. In addition, the pitch angle of electrons with higher energies is essentially field-aligned, and the fluxes of electrons with lower energies rapidly increase. It implies that the coexistence of different magnetic field line topologies inside the ion-scale flux rope is responsible for the observed electron pitch angle distributions within the flux rope. These results can be useful for understanding the reconnection process in three-dimension and the generation mechanism of the ion-scale flux rope.