[论文解读] On the Cause of Supra-Arcade Downflows

P. A. Cassak, J. F. Drake+4 more|arXiv (Cornell University)|Jul 15, 2013

Solar and Space Plasma Dynamics被引用 1

一句话总结

本文提出， supra-arcade downflows（SADs）是由具有导向磁场的、空间局域化且时间连续的磁重联喷流引起的，这些喷流在分层的太阳日冕中刻蚀出细长的低密度通道。三维模拟证实，此类磁重联能维持喷流的准直性，从而解释了太阳耀斑中观测到的持续且狭窄的SAD形态。

ABSTRACT

A model of supra-arcade downflows (SADs), dark low density regions also known as tadpoles that propagate sunward during solar flares, is presented. It is argued that the regions of low density are flow channels carved by sunward-directed outflow jets from reconnection. The solar corona is stratified, so the flare site is populated by a lower density plasma than that in the underlying arcade. As the jets penetrate the arcade, they carve out regions of depleted plasma density which appear as SADs. The present interpretation differs from previous models in that reconnection is localized in space but not in time. Reconnection is continuous in time to explain why SADs are not filled in from behind as they would if they were caused by isolated descending flux tubes or the wakes behind them due to temporally bursty reconnection. Reconnection is localized in space because outflow jets in standard two-dimensional reconnection models expand in the normal (inflow) direction with distance from the reconnection site, which would not produce thin SADs as seen in observations. On the contrary, outflow jets in spatially localized three-dimensional reconnection with an out-of-plane (guide) magnetic field expand primarily in the out-of-plane direction and remain collimated in the normal direction, which is consistent with observed SADs being thin. Two-dimensional proof-of-principle simulations of reconnection with an out-of-plane (guide) magnetic field confirm the creation of SAD-like depletion regions and the necessity of density stratification. Three-dimensional simulations confirm that localized reconnection remains collimated.

研究动机与目标

• 解决长期以来关于为何SADs保持狭窄且持久而非从后方填充的谜题。
• 解释太阳耀斑中观测到的SADs形态稳定性和狭窄宽度。
• 挑战以往将SADs归因于孤立磁通管下降或瞬时爆发性磁重联的模型。
• 研究导向磁场在维持准直磁重联喷流中的作用。

提出的方法

• 构建二维原理验证模型，模拟具有导向磁场的磁重联，以展示类似SAD的密度缺失的形成。
• 在日冕中引入等离子体密度分层，其中耀斑区域的密度低于下方的弧形结构。
• 利用三维模拟检验：局域化磁重联配合导向场是否能在法向方向维持喷流的准直性。
• 将磁重联建模为时间上连续但空间上局域化，以防止SADs从后方被填充。
• 分析喷流在出平面方向和法向方向的膨胀动力学，以评估其准直特性。
• 将模拟得到的SAD形态与观测结果对比，验证模型与细长、向太阳方向传播的特征的一致性。

实验结果

研究问题

• RQ1 为何SADs保持狭窄且持久，而非被周围等离子体填充？
• RQ2 导向磁场的存在如何影响三维空间中磁重联喷流的准直性？
• RQ3 连续磁重联能否解释SADs的持续存在而无需时间间隙？
• RQ4 为何标准的二维磁重联模型无法再现观测到的SADs的狭窄宽度？
• RQ5 日冕等离子体分层在SADs的形成与可见性中起什么作用？

主要发现

• SADs由空间局域化磁重联产生的向太阳方向喷流形成，这些喷流在分层日冕中刻蚀出低密度通道。
• 时间上连续的磁重联可防止SADs从后方被填充，与持久性特征的观测结果一致。
• 具有导向磁场的磁重联产生准直喷流，其在法向方向保持狭窄，与观测到的SAD形态相符。
• 二维模拟中，具有导向磁场的磁重联可成功生成类似SAD的密度缺失，验证了核心机制。
• 三维模拟证实，局域化磁重联配合导向磁场能维持准直性，从而解释了观测到的SADs的狭窄性。
• 该模型解释了为何二维磁重联模型无法产生狭窄SADs——原因在于法向方向存在非物理解理的膨胀。