Three‐Dimensional X‐line Spreading in Asymmetric Magnetic Reconnection

• Published in: Journal of geophysical research. Space physics Vol. 125; no. 2
• Main Authors: Li, Tak Chu, Liu, Yi‐Hsin, Hesse, Michael, Zou, Ying
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-02-2020
• Subjects: Alfven waves; Asymmetry; collisionless plasmas; Computer simulation; Current sheets; dayside magnetopause; kinetic simulations; Magnetic reconnection; Magnetohydrodynamics; Magnetopause; Numerical simulations; Simulation; Tearing; Thickness
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2019JA027094

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
VAg(sub‐Alfvénic spreading). In simulations with a sub‐ion‐scale CS, the X‐line spreads at
VAg instead (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
γ/Ωci≳O(1), 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.
Key Points
The continuous onset of reconnection is found to be important for 3‐D X‐line spreading in large‐scale particle‐in‐cell simulations
X‐line spreading at the Alfvén speed VAg (sub‐Alfvénic, species drifts) is observed in simulations with fast (slow) reconnection onset
The current sheet thickness and collisionless tearing growth rate play a key role in determining the spreading speeds