弱随机磁场中快速重联的数值试验
Numerical Tests of Fast Reconnection in Weakly Stochastic Magnetic Fields
摘要：
利用三维数值模拟研究了湍流对磁重联的影响。这是对Lazarian & Vishniac（1999）提出的弱湍流条件下快速磁重联模型的首次检验。该模型预测，弱湍流，一般存在于大多数天体物理系统，通过减少重联事件的横向尺度，并允许许多独立的通量重联事件同时发生，提高重联率。结果，重联速度变得与欧姆电阻率无关，并由湍流引起的磁场漂移决定。为了量化重联速度，我们使用直观的定义，即重联通量流入的速度，以及基于正式推导的解析表达式的更复杂的定义。我们的结果证实了Lazarian & Vishniac模型的预测。特别地，我们发现Vrec Pinj^（1/2），如模型所预测的。对于我们的一些模型，与预测的对注入尺度的线性依赖性相比，对注入尺度的依赖性比预期的要弱一些，即l^（3/4），这可能需要对模型进行一些改进，或者可能是由于激发区域的有限大小等效应。重联速度被发现依赖于预期的磁场漂移率，而不是在引导场的大小。在我们的模型中，我们看到没有依赖于引导场时，其强度是可比的重连组件。更重要的是，虽然在没有湍流的情况下，我们成功地再现了Sweet-Parker标度的重联，在湍流的存在下，我们没有观察到任何依赖于欧姆电阻率，确认我们的重联是快速的。
英文摘要：
We study the effects of turbulence on magnetic reconnection using 3D numerical simulations. This is the first attempt to test a model of fast magnetic reconnection in the presence of weak turbulence proposed by Lazarian & Vishniac (1999). This model predicts that weak turbulence, generically present in most of astrophysical systems, enhances the rate of reconnection by reducing the transverse scale for reconnection events and by allowing many independent flux reconnection events to occur simultaneously. As a result the reconnection speed becomes independent of Ohmic resistivity and is determined by the magnetic field wandering induced by turbulence. To quantify the reconnection speed we use both an intuitive definition, i.e. the speed of the reconnected flux inflow, as well as a more sophisticated definition based on a formally derived analytical expression. Our results confirm the predictions of the Lazarian & Vishniac model. In particular, we find that Vrec Pinj^(1/2), as predicted by the model. The dependence on the injection scale for some of our models is a bit weaker than expected, i.e. l^(3/4), compared to the predicted linear dependence on the injection scale, which may require some refinement of the model or may be due to the effects like finite size of the excitation region. The reconnection speed was found to depend on the expected rate of magnetic field wandering and not on the magnitude of the guide field. In our models, we see no dependence on the guide field when its strength is comparable to the reconnected component. More importantly, while in the absence of turbulence we successfully reproduce the Sweet-Parker scaling of reconnection, in the presence of turbulence we do not observe any dependence on Ohmic resistivity, confirming that our reconnection is fast.
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