The Nonorthogonal X-line in a Small Guide-field Reconnection Event in the Magnetotail

Abstract

Magnetic reconnection is a fundamental plasma process that has been studied with analytical theory, numerical simulations, in situ observations, and laboratory experiments for decades. The models that have been established to describe magnetic reconnection often assume a reconnection plane normal to the current sheet in which an antiparallel magnetic field annihilates. The annihilation points, also known as the X-points, form an x-line, which is believed to be perpendicular to the reconnection plane. Recently, a new study using Magnetospheric Multiscale mission observations has challenged our understanding of magnetic reconnection by providing evidence that the x-line is not necessarily orthogonal to the reconnection plane. In this study we report a second nonorthogonal x-line event with similar features as that in the previous case study, supporting that the sheared x-line phenomenon is not an aberrant event. We employ a detailed directional derivative analysis to identify the x-line direction and show that the in-plane reconnection characteristics are well maintained even with a nonorthogonal x-line. In addition, we find the x-line tends to follow the magnetic field on one side of the current sheet, which suggests an asymmetry across the current sheet. We discuss the possibility that the nonorthogonal x-line arises from an interplay between the two aspects of reconnection: the macroscopic magnetic field topology and microscopic particle kinetics.