Three‐Dimensional Magnetic Reconnection Spreading in Current Sheets of Non‐Uniform Thickness
Three‐Dimensional Magnetic Reconnection Spreading in Current Sheets of Non‐Uniform Thickness
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DOI：
10.1029/2022ja030999
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发表时间：
2023-03
期刊：
影响因子：
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通讯作者：
M. Arencibia;P. Cassak;M. Shay;J. Qiu;S. Petrinec;Haoming Liang
中科院分区：
文献类型：
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作者：
M. Arencibia;P. Cassak;M. Shay;J. Qiu;S. Petrinec;Haoming Liang
自然发生的和实验室环境中的磁重联通常在局部开始，并在垂直于重联平面的方向上拉长或扩散。以前的工作主要集中在均匀厚度的电流片上，对于这种电流片，反并联重联的预测扩展速度是电流载流子的局部速度。我们推导了电流片中无碰撞反平行重联的三维(3D)扩展的标度理论，该电流片的厚度沿平面外方向变化，既有从薄到厚的扩展，也有厚到薄的扩展。我们推导出了一个公式，用于计算具有给定厚度轮廓的当前薄板发生展开所需的时间。一个关键的结果是，当重新连接从较薄的区域扩展到较厚的区域时，较厚区域的扩展速度比Alfvén速度和局部电流载流子的速度都慢，这是由于电流片厚度的薄和厚的比率。这一点很重要，因为磁层和太阳观测之前测量到的扩散速度比之前预测的要慢，所以目前的机制可能解释了这一特征。我们通过使用3D双流体数值模拟的参数研究来证实这一理论。我们利用这个预测来计算地磁活动期间地球磁尾重联扩散的时间尺度。这一结果对于理解太阳耀斑中的重联扩散以及地球和其他行星的日侧磁层顶也具有潜在的重要意义。
Magnetic reconnection in naturally occurring and laboratory settings often begins locally and elongates, or spreads, in the direction perpendicular to the plane of reconnection. Previous work has largely focused on current sheets with a uniform thickness, for which the predicted spreading speed for anti‐parallel reconnection is the local speed of the current carriers. We derive a scaling theory of three‐dimensional (3D) spreading of collisionless anti‐parallel reconnection in a current sheet with its thickness varying in the out‐of‐plane direction, both for spreading from a thinner to thicker region and a thicker to thinner region. We derive an expression for calculating the time it takes for spreading to occur for a current sheet with a given profile of its thickness. A key result is that when reconnection spreads from a thinner to a thicker region, the spreading speed in the thicker region is slower than both the Alfvén speed and the speed of the local current carriers by a factor of the ratio of thin to thick current sheet thicknesses. This is important because magnetospheric and solar observations have previously measured the spreading speed to be slower than previously predicted, so the present mechanism might explain this feature. We confirm the theory via a parametric study using 3D two‐fluid numerical simulations. We use the prediction to calculate the time scale for reconnection spreading in Earth's magnetotail during geomagnetic activity. The results are also potentially important for understanding reconnection spreading in solar flares and the dayside magnetopause of Earth and other planets.