太阳物理研究组知识库

从理论和观测两个方面来介绍和讨论出现在太阳爆发过程中的磁重联电流片及其物理本质和动力学特征。首先介绍在理论研究和理论模型中,磁重联电流片是如何在爆发磁结构当中形成并发展的,对观测研究有什么指导意义。然后介绍观测工作是从哪几个方面对理论模型预测的电流片进行证认和研究的。第三,将介绍观测研究给出了哪些过去所没有能够预期的结果,这些结果对深入研究耀斑一CME电流片以及其中的磁重联过程的理论工作有什么重要的、挑战性的意义。第四,讨论最新的与此有关的理论研究和数值实验。最后,对未来的研究方向和重要课题进行综述和展望。

Magnetic reconnection is a fundamental process with a rich variety of aspects and applications in astrophysical, space, and laboratory plasmas. It is at the core of many dynamic phenomena in the universe, including solar eruptions, geomagnetic substorms, and tokamak disruption. Most of the universe is in the form of a plasma threaded by a magnetic field. When twisted or sheared, the field lines may reconnect rapidly, converting magnetic energy into heat and kinetic energy. Because these phenomena often occur in environments of very high electric conductivity, the process of energy conversion is usually confined to a small local region, such as an X-type neutral point, a current sheet, or a quasi-separatrix layer.  It is traditionally expected that the current sheet is too thin to be observable since its thickness is believed to be roughly the proton Larmor radius, which is about tens of meters in the coronal environment. This view is based on magnetic reconnection on small scales in the laboratory or on quasi-static process in space (with timescale of tens of hours or even a few days). This could be true as well in the solar eruption in the case that the classical Spitzer resistivity dominates the diffusion process.  However, CME/flare current sheets form and develop in a highly dynamical fashion in an eruptive process. Theoretical calculations indicate that the current sheet in major eruptive processes could evolve and extend in length at speed up to 102 km.s-s, and observational results suggest rapid evolution of the current sheet as well. In such a process, the scale, especially the thickness, of the current sheet should not be as simple as the Larmor radius of particles. Instead various plasma instabilities inevitably occur and play an important role in governing the scale of the current sheet.  Consistent with the above theoretical argument, the combination of observations from LASCO, EIT, UVCS on board SOHO detected directly the CME/flare current sheet soon after the basic framework of the catastrophe model of solar eruptions had constructed, and confirmed that the current sheet predicted by the model does exist and is observable inmajor eruptions. Plasma blobs flowing with the reconnection outflow inside the current sheet sunward and anti-sunward were observed subsequently, indicating that magnetic re- connection is occurring or has occurred. The reconnection inflow observed in the eruption marks the location and orientation of the current sheet, and helped estimate the apparent thickness of the current sheet for the first time. The result is astonishing, which shows that the CME/flare current sheet could be as thick as a few times 104 knfi A set of follow-ups by different instruments, one of them is even on the ground, and in different wavelengths consistently provided clear evidence bringing the sheet thickness to range from a few times 104 km to a few times 105 km[  The impact of projection effects on measurements surely exists, and causes the sheet to look thicker than it actually is. The geometric structure and relatively tenuous material of the current sheet yields that its size and emission are easily dominated by other large-scale and bright structures nearby. So detailed analyses showed that in the case that the current sheet is recognizable, the impact of the projection effects is limited, and only leads to the difference between the apparent and the true thicknesses less than a factor of 5. Therefore, the reason that causes the observed thick CME/flare current sheet is of physics, rather than optics.  The plasma blobs that successively appear in the CME/flare current sheet are very suggestive of various plasma instabilities inside the sheet. The tearing mode and the con- sequent plasmoid mode instabilities are the most important ones among them, which yields the fragmentation of the sheet and the fractal reconnection, resulting in extra significantly high resistivity that allows magnetic reconnection to progress at a reasonably high rate in the thick current sheet. Numerical experiments display clear features of fragmentation in- side the sheet, indicate that it is the fragmentation that speeds up the reconnection process tremendously, and further confirmed the turbulent or fractal behaviors of CME/flare current sheet and the associated reconnection process.