Transition from ion-coupled to electron-only reconnection: Basic physics and implications for plasma turbulence
Transition from ion-coupled to electron-only reconnection: Basic physics and implications for plasma turbulence
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DOI：
10.1063/1.5090403
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发表时间：
2019-01
影响因子：
2.2
通讯作者：
P. Pyakurel;M. Shay;T. Phan;W. Matthaeus;J. Drake;J. TenBarge;C. Haggerty;K. Klein;P. Cassak;T. Parashar;M. Swisdak;A. Chasapis
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作者：
P. Pyakurel;M. Shay;T. Phan;W. Matthaeus;J. Drake;J. TenBarge;C. Haggerty;K. Klein;P. Cassak;T. Parashar;M. Swisdak;A. Chasapis
使用动力学粒子模拟（PIC），我们模拟了适合磁鞘和太阳风的重连条件，即，等离子体β（气压与磁压之比）大于1和低磁剪切（强引导场）。改变模拟畴的大小，我们发现离子的响应变化很大。对于尺度与离子拉莫尔半径相当的重连区域，离子不响应重连动力学，导致具有非常大的准稳态重连率的“仅电子”重连。随着重联畴尺寸的增加，向更传统的“离子耦合”重联的过渡是逐渐的，当磁岛在法线方向上的宽度达到许多离子惯性长度时，离子在排气中被冻结。在此过渡期间，准稳态重联率下降，直到离子完全耦合，最终达到一个渐近值。在这个电子只有离子耦合过渡的排气宽度的离子流出速度的缩放被发现是一致的一个新的重新连接的场线的理论模型。为了使完全冻结的离子排气的离子流与重联Alfven速度相当，需要至少几个离子惯性长度的排气宽度。在湍流系统与磁泡之间发生的重联与波动，使用几何参数，我们估计，完全离子耦合重联需要至少几十个离子惯性长度的磁泡长度尺度。
Using kinetic particle-in-cell (PIC) simulations, we simulate reconnection conditions appropriate for the magnetosheath and solar wind, i.e., plasma beta (ratio of gas pressure to magnetic pressure) greater than 1 and low magnetic shear (strong guide field). Changing the simulation domain size, we find that the ion response varies greatly. For reconnecting regions with scales comparable to the ion Larmor radius, the ions do not respond to the reconnection dynamics leading to ''electron-only'' reconnection with very large quasi-steady reconnection rates. The transition to more traditional ''ion-coupled'' reconnection is gradual as the reconnection domain size increases, with the ions becoming frozen-in in the exhaust when the magnetic island width in the normal direction reaches many ion inertial lengths. During this transition, the quasi-steady reconnection rate decreases until the ions are fully coupled, ultimately reaching an asymptotic value. The scaling of the ion outflow velocity with exhaust width during this electron-only to ion-coupled transition is found to be consistent with a theoretical model of a newly reconnected field line. In order to have a fully frozen-in ion exhaust with ion flows comparable to the reconnection Alfv\'en speed, an exhaust width of at least several ion inertial lengths is needed. In turbulent systems with reconnection occurring between magnetic bubbles associated with fluctuations, using geometric arguments we estimate that fully ion-coupled reconnection requires magnetic bubble length scales of at least several tens of ion inertial lengths.