Torus-stable zone above starspots
Torus-stable zone above starspots
复制标题
星斑上方的环面稳定区
DOI：
10.1093/mnras/stab3249
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发表时间：
2021
影响因子：
4.8
通讯作者：
DeRosa, Marc L.
中科院分区：
文献类型：
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作者：
Török, Tibor;DeRosa, Marc L.
虽然强烈的太阳耀斑几乎总是伴随着日冕物质抛射（CME），但关于恒星CME的报道很少，尽管经常检测到恒星“超级耀斑”。磁绳环的不稳定性被认为是太阳日冕物质抛射的主要驱动机制之一。抑制环不稳定性，由于封闭的背景日冕磁场，降低足够缓慢的高度，可能有助于缺乏恒星日冕物质抛射检测。在这里，我们使用太阳磁场作为模板来估计的垂直范围内，这个“环面稳定区”（TSZ）以上的恒星活动区。对于一个理想化的势场模型，包括当地的双极（模仿一对星点）和全球偶极子的领域，我们表明，TSZ的上限增加的双极的大小，偶极子的强度，和源表面半径的冠状场成为径向。TSZ的边界取决于点和偶极子磁场之间的相互作用，这分别提供了局部和全局尺度的限制。它们的范围从双极的一半大小到恒星半径的一个重要部分。对于较小的点和中间偶极场，二次TSZ出现在较高的海拔高度，这可能会增加“失败的喷发”的可能性。我们的研究结果表明，低表观CME发生率的冷星，至少部分是由于存在扩展的TSZ。
Whilst intense solar flares are almost always accompanied by a coronal mass ejection (CME), reports on stellar CMEs are rare, despite the frequent detection of stellar ‘super flares’. The torus instability of magnetic flux ropes is believed to be one of the main driving mechanisms of solar CMEs. Suppression of the torus instability, due to a confining background coronal magnetic field that decreases sufficiently slowly with height, may contribute to the lack of stellar CME detection. Here, we use the solar magnetic field as a template to estimate the vertical extent of this ‘torus-stable zone’ (TSZ) above a stellar active region. For an idealized potential field model comprising the fields of a local bipole (mimicking a pair of starspots) and a global dipole, we show that the upper bound of the TSZ increases with the bipole size, the dipole strength, and the source surface radius where the coronal field becomes radial. The boundaries of the TSZ depend on the interplay between the spots’ and the dipole’s magnetic fields, which provide the local- and global-scale confinement, respectively. They range from about half the bipole size to a significant fraction of the stellar radius. For smaller spots and an intermediate dipole field, a secondary TSZ arises at a higher altitude, which may increase the likelihood of ‘failed eruptions’. Our results suggest that the low apparent CME occurrence rate on cool stars is, at least partially, due to the presence of extended TSZs.