螺环不稳定性对日冕物质抛射至关重要吗?

Is torus instability crucial for solar coronal mass ejections?

作为主序星的太阳,其内部结构大体是稳定的.核心区的氢聚变反应为太阳提供了持续而比较恒定的能源,维持其3.828×10^26 W的电磁辐射、8.8×10^24 W的中微子辐射,以及往外输运的太阳风.然而,由于原初磁场[1]以及太阳表面以下0.3倍半径范围内(即对流区)较差自转的存在,在发电机过程[2]及磁扩散的共同作用下,对流区底部的磁场发生着周期性的变化.当其磁场强到一定程度,便会经由太阳表面而浮现到太阳大气中,并导致各种空间尺度和不同能量的爆发现象,如太阳耀斑、暗条爆发、日冕物质抛射等.其中,日冕物质抛射是尺度最大的剧烈爆发现象,质量高达10^11~10^13 kg的物质携带着磁场以几十到4000 km/s的速度在行星际空间传播[3,4].当它掠过地球时,会产生地磁扰动,强烈时严重影响人造卫星的正常运行、短波通讯、导航以及高纬度的电网.因此,对日冕物质抛射的触发及爆发机制的研究非常重要.这种研究不但对发生在其他恒星乃至吸积盘上的爆发过程有借鉴作用,也能够为预报地球周围的空间环境可能出现的灾害性扰动提供物理基础.国内很多研究组都在从事日冕物质抛射方面的研究工作[5].

Coronal mass ejections (CMEs) are the most violent eruptions in the solar atmosphere. What are the progenitors? How are the progenitors triggered to erupt? How are CMEs accelerated and how do they propagate in the interplanetary space? All these issues still remain elusive. In particular, a lot of efforts have been taken to the triggering mechanisms. From both theoretical and observational points of view, a CME progenitor can be triggered from a metastable state to eruption through either a resistive process, e.g., magnetic reconnection, or ideal magnetohydrodynamic (MHD) instabilities, e.g., kink and torus instabilities. About thirty years ago, it was once thought that CMEs can be due to an ideal MHD process, which opens up the initially closed magnetic loops, and the ensuing magnetic reconnection leads to a solar flare, which is dispensible for the CMEs. Considering the Aly-Sturrock constraint and the intimate correlation between CMEs and flares, it was then realized that magnetic reconnection may play a crucial role later. However, in the past fourteen years, there was an increasing trend to emphasize torus instability in CMEs, not only for their triggering, but also for the final eruption. On one hand, we believe that torus instability, as well as kink instability and magnetic reconnection, is one of the possible trigger mechanisms for CMEs;on the other hand, we are suspicious of some papers on how the torus instability was identified. Our main arguments are summarized as follows: (1) Torus instability is only one of the trigger mechanisms. Other mechanisms may work as well, and sometimes several mechanisms may be at work simultaneously. Therefore, it is not surprising at all that a CME erupts while the criterion of torus instability is not satisfied. (2) Torus instability can serve as a trigger mechanism, but presumably not a driven mechanism. Whether a CME can erupt or not depends on both external conditions, e.g., the background magnetic field, and internal conditions, e.g., the nonpotentiality of the core magnetic field in the source region and how fast the reconnection proceeds. Based on the Aly-Sturrock constraint, a line-tied flux rope can never erupt without reconnection or mass drainage even when the background magnetic field satisfies the criterion of torus instability everywhere from low corona all the way to the interplanetary space. When some papers claimed the key role of torus instability, magnetic reconnection was happening as well, which makes their conclusions unconvincing. So, it will be critical to distinguish resistive processes from ideal MHD instabilities in future studies. (3) When studying the torus instability, it is better to calculate the 3-dimensional distribution of the decay index of the background magnetic field. Checking the decay index along one direction is not sufficient. (4) When studying the torus instability, it might be meaningless to examine the decay index only at the apex of a flux rope. A local loss of equilibrium never means the catastrophe of a whole structure. We need to take in account the distribution of the decay index along the major part of the flux rope in the source region.