Oblique Ion-Scale Magnetotail Flux Ropes Generated by Secondary Tearing Modes

摘要

We examine Magnetospheric Multiscale observations of three ion-scale magnetic flux ropes (diameter ~6–9 ion inertial lengths) in the Earth’s magnetotail reconnection diffusion region with a small guide field. During the event, the electron reconnection exhaust reversed direction together with the reversal of normal magnetic field component. After that, the three ion-scale flux ropes were observed near the ion diffusion region, in which flat-top distributions were found at the field-aligned electrons (<2 keV). The axis orientation and motion velocity of the flux ropes are determined by minimum variance analysis of the electric field component along the flux rope axis, using single-spacecraft measurements. Results show that the three flux rope axes form 13°, 55°, and 55° to the guide field direction and are mostly tilted toward the direction of the reconnecting field. These results are also validated by multiple-spacecraft analysis methods. It is found that the tilted angles agree reasonably well with the predicted angles for secondary tearing modes, suggesting that these ion-scale flux ropes are likely generated by secondary tearing instabilities as those recently found in the three-dimensional kinetic simulations of turbulent magnetic reconnection.