Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08544, USA.
Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, USA.
Nature. 2015 Dec 24;528(7583):526-9. doi: 10.1038/nature16188.
Coronal mass ejections are solar eruptions driven by a sudden release of magnetic energy stored in the Sun's corona. In many cases, this magnetic energy is stored in long-lived, arched structures called magnetic flux ropes. When a flux rope destabilizes, it can either erupt and produce a coronal mass ejection or fail and collapse back towards the Sun. The prevailing belief is that the outcome of a given event is determined by a magnetohydrodynamic force imbalance called the torus instability. This belief is challenged, however, by observations indicating that torus-unstable flux ropes sometimes fail to erupt. This contradiction has not yet been resolved because of a lack of coronal magnetic field measurements and the limitations of idealized numerical modelling. Here we report the results of a laboratory experiment that reveal a previously unknown eruption criterion below which torus-unstable flux ropes fail to erupt. We find that such 'failed torus' events occur when the guide magnetic field (that is, the ambient field that runs toroidally along the flux rope) is strong enough to prevent the flux rope from kinking. Under these conditions, the guide field interacts with electric currents in the flux rope to produce a dynamic toroidal field tension force that halts the eruption. This magnetic tension force is missing from existing eruption models, which is why such models cannot explain or predict failed torus events.
日冕物质抛射是由储存在太阳日冕中的磁能突然释放驱动的太阳爆发。在许多情况下，这种磁能储存在称为磁通量绳的长寿命、拱形结构中。当通量绳失稳时，它可能会爆发并产生日冕物质抛射，也可能会失败并回落到太阳。目前的观点认为，给定事件的结果是由称为环量不稳定性的磁流体动力力不平衡决定的。然而，观测表明，环量不稳定的通量绳有时不会爆发，这一观点受到了挑战。由于缺乏日冕磁场测量和理想数值模拟的限制，这一矛盾尚未得到解决。在这里，我们报告了一项实验室实验的结果，该实验揭示了一个以前未知的爆发判据，低于该判据，环量不稳定的通量绳就无法爆发。我们发现，当引导磁场（即沿通量绳环形运行的环境场）足够强，阻止通量绳扭结时，就会发生这种“失败的环量”事件。在这些条件下，引导场与通量绳中的电流相互作用，产生阻止爆发的动态环形磁场张力。现有爆发模型中缺少这种磁张力，这就是为什么这些模型无法解释或预测失败的环量事件。
