Three-Dimensional X-line Spreading in Asymmetric Magnetic Reconnection

Abstract

The spreading of the X-line out of the reconnection plane under a strong guide field is investigated using large-scale three-dimensional (3D) particle-in-cell (PIC) simulations in asymmetric magnetic reconnection. A simulation with a thick, ion-scale equilibrium current sheet (CS) reveals that the X-line spreads at the ambient ion/electron drift speeds, significantly slower than the Alfvén speed based on the guide field $V_{Ag}$. Additional simulations with a thinner, sub-ion-scale CS show that the X-line spreads at $V_{Ag}$ (Alfvénic spreading), much higher than the ambient species drifts. An Alfvénic signal consistent with kinetic Alfvén waves develops and propagates, leading to CS thinning and extending, which then results in reconnection onset. The continuous onset of reconnection in the signal propagation direction manifests as Alfvénic X-line spreading. The strong dependence on the CS thickness of the spreading speeds, and the X-line orientation 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 $\gamma /{\Omega }_{ci}\gtrsim \mathcal{O}(1)$, Alfvénic X-line spreading can take place. Our results compare favorably with a number of numerical simulations and recent magnetopause observations. A key implications is that the magnetopause CS is typically too thick for Alfvénic X-line spreading to effectively take place.

arXiv commentsubmitted to JGR

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Cited by in corpus (8)

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• Identification of active magnetic reconnection using magnetic flux transport in plasma turbulence
• Relativistic Asymmetric Reconnection
• Scaling of magnetic reconnection with a limited x-line extent
• Radial velocity map of solar wind transients in the field of view of STEREO/HI1 on 3 and 4 April 2010
• Extended magnetic reconnection in kinetic plasma turbulence
• Scaling Theory of 3D Magnetic Reconnection Spreading
• Three-Dimensional Magnetic Reconnection Spreading in Current Sheets of Non-Uniform Thickness