部分电离等离子体中由等离子体团介导的二维磁重联
Plasmoid-Mediated 2D Magnetic Reconnection in Partially Ionized Plasmas
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中文总结 AI 辅助
本研究通过高分辨率二维双流体模拟，揭示部分电离等离子体中中性-离子解耦改变等离子体团层级、提升重联率的机制，补充了高伦德奎斯特数下相关非线性行为的认知。
中文摘要 AI 辅助
部分电离等离子体中的磁重联是能量释放的重要途径。虽然等离子体团不稳定性在二维完全电离等离子体中已有明确表征，但在以中性粒子为主的等离子体环境下，其在非线性高伦德奎斯特数（$S = 10^5$）区域的行为仍缺乏清晰认知。我们开展了高分辨率（$16384 \times 4096$ 格点）的二维双流体（离子+中性粒子）Harris片重联模拟，上游等离子体β值 $β= 2$，对比了完全电离与部分电离（电离分数 $ξ= 10^{-1}$ 和 $10^{-2}$）两种情形。中性粒子与离子的解耦会加速线性撕裂阶段并改变等离子体团层级结构：完全电离情形下占主导的大尺度“巨型”等离子体团受到抑制，电流片反而碎裂为密集的亚尺度等离子体团链。在中性-离子解耦尺度 $\rm \nell_{dec}$ 以下，离子聚集在等离子体团内，峰值过密度 $ρ_i/ρ_{i,0}$ 约为10（$ξ= 10^{-1}$ 时）和 $3-5\times10^{3}$（$ξ= 10^{-2}$ 时），而中性粒子分布则相对平滑。这种局部堆积会提升电离分数，使等离子体团内的两种流体重新耦合。通过重联位点处的面外电场测量，$ξ= 10^{-2}$ 情形下的重联率 $R_{\rm rec}\napprox0.01$，而 $ξ= 10^{-1}$ 情形下重联率升至约0.02，发生明显聚并时进一步升至0.035。在 $ξ= 10^{-2}$ 情形下，双极漂移驱动离子以约 $0.5\nv_{A,0}$ 的速度快速流入重联层，远高于约 $0.1\rv_{A,0}$ 的中性粒子流入速度。其中 $v_{A,0}$ 为上游总阿尔芬速度。
英文摘要
Magnetic reconnection in partially ionized plasmas is an important channel for energy release. While the plasmoid instability is well characterized in 2D fully ionized plasmas, its behavior in the presence of neutral-dominated plasma remains poorly understood in the nonlinear, high-Lundquist-number ($S = 10^5$) regime. We present high-resolution ($16384 \times 4096$ cells) two-dimensional two-fluid (ion $+$ neutral) simulations of Harris-sheet reconnection with upstream plasma beta $β= 2$, comparing fully ionized and partially ionized (ionization fraction $ξ= 10^{-1}$ and $10^{-2}$) regimes. Neutral-ion decoupling accelerates the linear tearing stage and alters the plasmoid hierarchy: the large-scale ``monster'' plasmoid that dominates the fully ionized case is suppressed, and the sheet instead fragments into a dense chain of sub-scale plasmoids. Below the neutral-ion decoupling scale $\ell_{\rm dec}$, ions concentrate into the plasmoids, reaching peak overdensities $ρ_i/ρ_{i,0} \approx 10$ ($ξ= 10^{-1}$) and $3-5\times10^{3}$ ($ξ= 10^{-2}$), while the neutrals remain comparatively smooth. This local pile-up raises the ionization fraction and recouples the two fluids within the plasmoids. Measured from the out-of-plane electric field at the reconnection sites, the reconnection rate in the $ξ= 10^{-2}$ case achieves $R_{\rm rec}\approx0.01$, whereas the $ξ= 10^{-1}$ case rises to a rate $\approx0.02$ and further $0.035$ when apparent coalescence occurs. In the $ξ= 10^{-2}$ case, the ambipolar drift drives a rapid ion inflow $\sim0.5\,v_{A,0}$ into the layer at the same reconnection sites, far above the neutral inflow velocity $\sim0.1\,v_{A,0}$. Here, $v_{A,0}$ is the upstream total Alfvén speed.
