skip to main content

This content will become publicly available on January 17, 2025

Title: Determining the Orientation of a Magnetic Reconnection X Line and Implications for a 2D Coordinate System

AnLMNcoordinate system for magnetic reconnection events is sometimes determined by definingNas the direction of the gradient across the current sheet andLas the direction of maximum variance of the magnetic field. The third direction,M, is often assumed to be the direction of zero gradient, and thus the orientation of the X line. But when there is a guide field, the X line direction may have a significant component in the L direction defined in this way. For a 2D description, a coordinate system describing such an event would preferably be defined using a different coordinate directionM′ oriented along the X line. Here we use a 3D particle‐in‐cell simulation to show that the X line is oriented approximately along the direction bisecting the asymptotic magnetic field directions on the two sides of the current sheet. We describe two possible ways to determine the orientation of the X line from spacecraft data, one using the minimum gradient direction from Minimum Directional Derivative analysis at distances of the order of the current sheet thickness from the X line, and another using the bisection direction based on the asymptotic magnetic fields outside the current sheet. We discuss conditions for validity of these estimates, and we illustrate these conditions using several Magnetospheric Multiscale (MMS) events. We also show that intersection of a flux rope due to secondary reconnection with the primary X line can destroy invariance along the X line and negate the validity of a two‐dimensional description.

more » « less
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Journal of Geophysical Research: Space Physics
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. 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.

    more » « less
  2. 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
  3. Abstract

    The current sheet structure and ion behaviors in a magnetotail reconnection diffusion region are investigated. The multispacecraft analysis suggests a corrugated current sheet structure, interpreted as due to a flapping motion that propagates along geocentric solar magnetospheric along the +ydirection in the Geocentric Solar Magnetospheric (GSM) coordinate. The electric field (E) and ion distributions have similarities with those in a planar current sheet. Energetic ions move along the current direction, suggesting the acceleration by the observed reconnectionEduring the meandering motion. Counterstreaming ions along the current sheet normal suggest the acceleration by the HallEthat is observed to be the dominant component. However, at certain locations,Eand counterstreaming ions significantly deviate from the local normal direction, and more than one pair of counterstreaming populations exist, possibly because the corrugated current sheet enables ions entering the current sheet at different locations with different velocities to mix together.

    more » « less
  4. Abstract

    Three‐dimensional X‐line spreading is important for the coupling between global dynamics and local kinetic physics of magnetic reconnection. Using large‐scale 3‐D particle‐in‐cell simulations, we investigate the spreading of the X‐line out of the reconnection plane under a strong guide field in asymmetric reconnection. The X‐line spreading speed depends strongly on the equilibrium current sheet thickness. In a simulation with a thick, ion‐scale equilibrium current sheet (CS), the X‐line spreads at the ambient species drift speeds, which are significantly lower than the Alfvén speed based on the guide field(sub‐Alfvénic spreading). In simulations with a sub‐ion‐scale CS, the X‐line spreads atinstead (Alfvénic spreading). An Alfvénic signal consistent with kinetic Alfvén waves develops and propagates, leading to CS thinning and extending, which ultimately causes reconnection onset. The continuous onset of reconnection along the propagation direction of the signal manifests as Alfvénic X‐line spreading. The strong dependence on the CS thickness of the spreading speeds and the orientation of the X‐line are consistent with the collisionless tearing instability. Our simulations indicate that when the collisionless tearing growth is sufficiently strong in a thinner CS such that, Alfvénic X‐line spreading can effectively take place. Our results compare favorably with a number of numerical simulations and recent magnetopause observations. An important implication of this work is that the magnetopause CS is typically too thick for the X‐line to spread at the Alfvén speed.

    more » « less
  5. Abstract

    We report an event of two‐satellite measurements of simultaneous earthward and tailward fast flows of ~500 km/s in the midtail atX ~ −63 REand evaluate magnetic reconnection as a responsible mechanism by comparing the observations with a particle‐in‐cell (PIC) simulation. The two satellites were near midnight separated mainly along theXdirection by ~5 RE. As they moved across the current sheet from the northern to southern lobes, the one closer to the Earth crossed the x line with fast flows changing from tailward to earthward, while the other one simultaneously observed tailward flows. The observed plasma and fields showed several key reconnection signatures, including the Walén relation, the fast reconnection rate of ~0.1, the Hall magnetic and electric fields, and counterstreaming electrons in the separatrix, indicating the fast flow was the reconnection exhaust. The observed temporal variations of flow speeds and magnetic fields suggested that the x line was moving tailward to a location between the two satellites and the exhaust was moving up and down. Within the exhaust, plasma pressure was highly anisotropic, and the current sheet can be unstable to the mirror, ion cyclotron, and firehose instabilities. Current sheet flapping and enhanced compressional waves near proton's local gyro frequencies were observed around the current sheet. Comparing with the PIC simulation suggests that the waves were mainly a result of oblique firehose instability.

    more » « less