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Abstract We present observations that suggest the X-line of guide-field magnetic reconnection is not necessarily orthogonal to the plane in which magnetic reconnection is occurring. The plane of magnetic reconnection is often referred to as theL–Nplane, whereLis the direction of the reversing and reconnecting magnetic field andNis normal to the current sheet. The X-line is often assumed to be orthogonal to theL–Nplane (defined as theM-direction) in the majority of theoretical studies and numerical simulations. The four-satellite Magnetospheric Multiscale (MMS) mission, however, observes a guide-field magnetic reconnection event in Earth’s magnetotail in which the X-line may be oblique to theL–Nplane. This finding is somewhat opportune as two of the MMS satellites at the sameNlocation report nearly identical observations with no significant time delays in the electron diffusion region (EDR) even though they have substantial separation inL. A minimum directional derivative analysis suggests that the X-line is between 40° and 60° fromM, adding support that the X-line is oblique. Furthermore, the measured ion velocity is inconsistent with the apparent motion of the MMS spacecraft in theL-direction through the EDR, which can be resolved if one assumes a shear in theL–Nplane and motion in theM-direction. A nonorthogonal X-line, if somewhat common, would call for revisiting theory and simulations of guide-field magnetic reconnection, reexamination of how the reconnection electric field is supported in the EDR, and reconsidering the large-scale geometry of the X-line.
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Orientation and Stability of Asymmetric Magnetic Reconnection X Line
The orientation and stability of the reconnection x line in asymmetric geometry is studied using three‐dimensional (3‐D) particle‐in‐cell simulations. We initiate reconnection at the center of a large simulation domain to minimize the boundary effect. The resulting x line has sufficient freedom to develop along an optimal orientation, and it remains laminar. Companion 2‐D simulations indicate that this x line orientation maximizes the reconnection rate. The divergence of the nongyrotropic pressure tensor breaks the frozen‐in condition, consistent with its 2‐D counterpart. We then design 3‐D simulations with one dimension being short to fix the x line orientation but long enough to allow the growth of the fastest growing oblique tearing modes. This numerical experiment suggests that reconnection tends to radiate secondary oblique tearing modes if it is externally (globally) forced to proceed along an orientation not favored by the local physics. The development of oblique structure easily leads to turbulence inside small periodic systems.
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- Award ID(s):
- 1757599
- PAR ID:
- 10076494
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 123
- Issue:
- 6
- ISSN:
- 2169-9380
- Page Range / eLocation ID:
- 4908-4920
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1029/2018JA025410
