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

We examine Magnetospheric Multiscale observations of three ion-scale magnetic flux ropes (diameter similar to 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 degrees, 55 degrees, and 55 degrees 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.