Collisionless Magnetic Reconnection

这个项目将集中在理解弱碰撞系统中重联的开始以及湍流和异常电阻率对磁重联的影响。在弱碰撞系统中，对于给定的等离子体碰撞度，存在两种重联解，慢Sweet-Parker重联解和快Hall重联解。在临界电阻率以下，Sweet-Parker解消失，重连率突然增加许多数量级，达到快速霍尔率。到目前为止，这种灾难性转变的建模仅限于反平行重联的情况。该项目将通过引入引导磁场来扩展过渡的建模。此外，还将采用一种基于物理学的半分析过渡模型。分析模型将与数值模拟的结果进行比较。专门的实验室重联实验和地球磁层卫星观测有大量证据表明，在无碰撞磁重联过程中，x线附近和磁分界线沿着驱动的强电流区域是湍流。湍流的存在或不存在与局部电子β参数（等离子体压力除以磁压力的比值）有关。如果电子β较低，携带大部分重联驱动电流的电子的流动速度将超过局部电子热速度。在这些条件下，将出现流动不稳定性，产生异常电阻率，打破磁通冻结条件。该项目将研究具有不同电子β值的二维模拟，并将测试电子分布的稳定性和流动不稳定性。一个关键的问题是，局部电子加热是否足够强，以抑制这种不稳定性。在已经建立不稳定性的区域，将完成这种湍流发展的三维模拟，以确定所产生的湍流和异常电阻率的β依赖性。

This project will focus on understanding the onset of reconnection in weakly collisional systems and the impact of turbulence and anomalous resistivity on magnetic reconnection. In weakly collisional systems, two reconnection solutions exist for a given plasma collisionality, a slow Sweet-Parker solution and a fast Hall reconnection solution. Below a critical resistivity the Sweet-Parker solution disappears and the reconnection rate abruptly increases by many orders of magnitude to the fast Hall rate. Up to this point the modeling of this catastrophic transition has been limited to the case of anti-parallel reconnection. This project will extend the modeling of the transition by including a guide magnetic field. In addition, a physics-based, semi-analytic model of the transition will be pursued. The analytic model will be compared with the results of numerical simulations. There is substantial evidence from dedicated laboratory reconnection experiments and satellite observations in the Earth's magnetosphere that the regions of intense current that are driven near the x-line and along the magnetic separatrices during collisionless magnetic reconnection are turbulent. The presence or absence of turbulence is linked to the local electron beta parameter (the ratio of the plasma pressure divided by the magnetic pressure). If the electron beta is low, the streaming velocity of electrons, which carry most of the reconnection driven current, will exceed the local electron thermal velocity. Under these conditions, streaming instabilities will arise, producing anomalous resistivity that breaks the frozen-in-flux condition. The project will examine 2-D simulations with differing values of the electron beta and the electron distributions will be tested for stability to streaming instabilities. A critical question is whether local electron heating is strong enough to inhibit such instabilities. In regions where instability has been established 3-D simulations of the development of this turbulence will be completed to determine the beta dependence of the resulting turbulence and anomalous resistivity.