Revisiting the MRX Electron Current Sheet Width with Semi-Collisional Kinetic Simulations at Hydrogen Mass Ratio
Abstract
For over eighteen years, the electron current sheet measured in the Magnetic Reconnection Experiment (MRX) has stood a factor of 2--5 wider than predicted by MRX-like kinetic simulations, and the measured electron force balance has not closed with the classical terms alone. As a consequence, the dominant nonideal terms responsible for breaking the frozen-in condition have remained unexplained. Here, two-dimensional kinetic simulations with binary Coulomb collisions are performed in a cylindrical geometry representative of MRX, at the realistic hydrogen mass ratio mi/me = 1836 and at MRX-relevant collisionality. For the first time, these simulations reproduce the measured electron current sheet half-width. The simulated value, δBT = 0.744 ± 0.054~cm or 6.26 ± 0.45 electron skin depths (de), lies within the experimental range of 5.5--7.5~de. The electron force balance closes through the classical channels alone: the pressure-tensor divergence supports 76\% of the nonideal electric field and collisional friction the remainder. The historical force-balance deficit reappears only when the simulated layer is sampled at the experimental 3~cm outflow resolution, suggesting that the deficit reflects probe resolution rather than anomalous dissipation. Beyond this reproduction, an analytic model of the layer width is developed that orders the meandering electrons by the coherence of their orbits against collisions. In this model, the Dreicer ratio ED/|Ey| selects the electrons whose current-carrying motion survives, and the resulting width prediction brackets the measured values across a wide collisionality scan. A discrepancy remains in the width normalized to the local electron gyroradius (ρe), whose measured value lies a factor of 3--6 above both the model and the simulations.
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BibTeX
@misc{son2026revisiting,
title = {{Revisiting the MRX Electron Current Sheet Width with Semi-Collisional Kinetic Simulations at Hydrogen Mass Ratio}},
author = {Sung Hyun Son and Adam Stanier and William Daughton and Jongsoo Yoo and Tongnyeol Rhee and Hantao Ji},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.01793},
eprint = {2609.01793},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.01793},
note = {CC BY-NC-ND 4.0},
}
RIS
TY - UNPB
TI - Revisiting the MRX Electron Current Sheet Width with Semi-Collisional Kinetic Simulations at Hydrogen Mass Ratio
AU - Sung Hyun Son
AU - Adam Stanier
AU - William Daughton
AU - Jongsoo Yoo
AU - Tongnyeol Rhee
AU - Hantao Ji
PY - 2026
DA - 2026/09/01
PB - arXiv
DO - 10.48550/arxiv.2609.01793
UR - https://arxiv.org/abs/2609.01793
AB - For over eighteen years, the electron current sheet measured in the Magnetic Reconnection Experiment (MRX) has stood a factor of 2--5 wider than predicted by MRX-like kinetic simulations, and the measured electron force balance has not closed with the classical terms alone. As a consequence, the dominant nonideal terms responsible for breaking the frozen-in condition have remained unexplained. Here, two-dimensional kinetic simulations with binary Coulomb collisions are performed in a cylindrical geometry representative of MRX, at the realistic hydrogen mass ratio m_i/m_e = 1836 and at MRX-relevant collisionality. For the first time, these simulations reproduce the measured electron current sheet half-width. The simulated value, δ_BT = 0.744 ± 0.054~cm or 6.26 ± 0.45 electron skin depths (d_e), lies within the experimental range of 5.5--7.5~d_e. The electron force balance closes through the classical channels alone: the pressure-tensor divergence supports 76\% of the nonideal electric field and collisional friction the remainder. The historical force-balance deficit reappears only when the simulated layer is sampled at the experimental 3~cm outflow resolution, suggesting that the deficit reflects probe resolution rather than anomalous dissipation. Beyond this reproduction, an analytic model of the layer width is developed that orders the meandering electrons by the coherence of their orbits against collisions. In this model, the Dreicer ratio E_D/|E_y| selects the electrons whose current-carrying motion survives, and the resulting width prediction brackets the measured values across a wide collisionality scan. A discrepancy remains in the width normalized to the local electron gyroradius (ρ_e), whose measured value lies a factor of 3--6 above both the model and the simulations.
ER -