A Two-step Magnetic Reconnection in a Confined X-class Flare in Solar Active Region 12673
A Two-step Magnetic Reconnection in a Confined X-class Flare in Solar Active Region 12673
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太阳活动区 12673 受限 X 级耀斑中的两步磁重联
DOI：
10.3847/1538-4357/aaf3b7
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发表时间：
2019
期刊：
影响因子：
--
通讯作者：
Fengsi Wei
中科院分区：
文献类型：
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作者：
Peng Zou;Chaowei Jiang;Xueshang Feng;Pingbing Zuo;Yulei Wang;Fengsi Wei
太阳耀斑通常与日冕爆发有关，但也有不爆发的受限耀斑，即使是一些x级耀斑。如此大的耀斑是如何发生的，以及为什么它们受到限制，人们仍然不太清楚。在这里，我们研究了2017年9月6日NOAA 12673中受限的X2.2耀斑。它表现出两次耀斑增亮，带有相当复杂的非典型带状。基于外推日冕磁场的拓扑分析，我们发现耀斑过程中存在两步磁重联过程。在耀斑之前，有一条磁通绳（MFR），一条腿植根于一个旋转的太阳黑子。与腿相邻的是磁性零点结构。太阳黑子驱动MFR膨胀，将磁通量推至零点，在那里首先触发重联。零点重接产生的扰动触发了第二次重接，即在绳索下方切割系绳重接。然而，这两次重新连接都没有引起喷发，因为绳子被捆扎的流体牢牢地缚住了。此外，我们将该耀斑与2小时后在同一地区爆发的X9.3耀斑进行了比较，两者具有相似的MFR结构。两者之间的关键区别在于，对于受限耀斑，MFR完全低于环面不稳定阈值，而对于爆发耀斑，MFR完全高于环面不稳定阈值。本研究提供了一个很好的证据，支持重连本身可能不能触发喷发，相反，MHD不稳定起着更重要的作用。
Solar flares are often associated with coronal eruptions, but there are confined ones without eruption, even for some X-class flares. How such large flares occurred and why they are confined are still not well understood. Here we studied a confined X2.2 flare in NOAA 12673 on 2017 September 6. It exhibits two episodes of flare brightening with rather complex, atypical ribbons. Based on topology analysis of extrapolated coronal magnetic field, we revealed that there is a two-step magnetic reconnection process during the flare. Prior to the flare, there is a magnetic flux rope (MFR) with one leg rooted in a rotating sunspot. Neighboring to the leg is a magnetic null-point structure. The sunspot drives the MFR to expand, pushing magnetic flux to the null point, and reconnection is first triggered there. The disturbance from the null-point reconnection triggers the second reconnection, i.e., a tether-cutting reconnection below the rope. However, these two reconnections failed to produce an eruption, because the rope is firmly held by its strapping flux. Furthermore, we compared this flare with an eruptive X9.3 flare in the same region with 2 hours later, which has a similar MFR configuration. The key difference between them is that, for the confined flare, the MFR is fully below the threshold of torus instability, while for the eruptive one, the MFR reaches entirely above the threshold. This study provides a good evidence supporting that reconnection alone may not be able to trigger eruption, rather, MHD instability plays a more important role.