Xing Chen, Cheng Xin, Ding Mingde
School of Astronomy and Space Science, Nanjing University, Nanjing 210046, China.
Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, China.
Innovation (Camb). 2020 Nov 5;1(3):100059. doi: 10.1016/j.xinn.2020.100059. eCollection 2020 Nov 25.
Coronal mass ejections (CMEs) are large-scale explosions of the coronal magnetic field. It is believed that magnetic reconnection significantly builds up the core structure of CMEs, a magnetic flux rope, during the eruption. However, the quantitative evolution of the flux rope, particularly its toroidal flux, is still unclear. In this paper, we study the evolution of the toroidal flux of the CME flux rope for four events. The toroidal flux is estimated as the magnetic flux in the footpoint region of the flux rope, which is identified by a method that simultaneously takes the coronal dimming and the hook of the flare ribbon into account. We find that the toroidal flux of the CME flux rope for all four events shows a two-phase evolution: a rapid increasing phase followed by a decreasing phase. We further compare the evolution of the toroidal flux with that of the Geostationary Operational Environmental Satellites soft X-ray flux and find that they are basically synchronous in time, except that the peak of the former is somewhat delayed. The results suggest that the toroidal flux of the CME flux rope may be first quickly built up by the reconnection mainly taking place in the sheared overlying field and then reduced by the reconnection among the twisted field lines within the flux rope, as enlightened by a recent 3D magnetohydrodynamic simulation of CMEs.
日冕物质抛射（CMEs）是日冕磁场的大规模爆发。人们认为，磁重联在喷发过程中显著构建了CMEs的核心结构——磁通量绳。然而，通量绳的定量演化，尤其是其环形通量，仍不清楚。在本文中，我们研究了四个事件中CME通量绳环形通量的演化。环形通量被估计为通量绳足点区域的磁通量，该区域通过一种同时考虑日冕暗化和耀斑带钩状结构的方法来识别。我们发现，所有四个事件中CME通量绳的环形通量都呈现出两阶段演化：快速增加阶段，随后是减少阶段。我们进一步将环形通量的演化与地球同步轨道环境卫星软X射线通量的演化进行比较，发现它们在时间上基本同步，只是前者的峰值略有延迟。结果表明，正如最近对CMEs的三维磁流体动力学模拟所启示的那样，CME通量绳的环形通量可能首先主要通过发生在剪切上层磁场中的重联快速建立起来，然后通过通量绳内扭曲磁力线之间的重联而减少。
