Orientation of x-lines in asymmetric magnetic reconnection -- application to Earth's magnetopause

Abstract:

At Earth's magnetopause, reconnection proceeds asymmetrically between the magnetosheath plasmas and magnetospheric plasmas. In an asymmetric configuration, it is unclear if there is a simple principle to determine the orientation of the x-line. The close deployment of MMS spacecraft cluster and the equipped capability of high-time resolution provide an invaluable chance to reconstruct the three-dimensional geometry of reconnection diffusion region [Denton et al. J. Geophys. Res. 117, A09201 (2012)], and maybe to accurately deduce the x-line orientation at Earth's magnetopause. In parallel to this observational effort, this work contains the theoretical foundation and prediction. We used three- and two- dimensional Particle-in-Cell simulations to study this fundamental problem. A spatially localized perturbation is employed to induce a single x-line, that has sufficient freedom to choose its orientation in three-dimensional systems. The effect of ion to electron mass ratio is investigated, and the x-line appears to bisect the magnetic shear angle across the current sheet in the large mass ratio limit. The deviation from the bisection angle in the lower mass ratio limit can be explained by the physics of tearing instability [JGR, accepted. arXiv:1504.03300]. The local physics control of the x-line orientation studied in this slab geometry could potentially interplay with global geometrical effects to determine the location of reconnection at Earth's magnetopause.