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| 太阳爆发过程中的大尺度磁重联电流片:理论和观测 | |
| 引用本文： | 吴宁,李燕,沈呈彩,林隽.太阳爆发过程中的大尺度磁重联电流片:理论和观测[J].天文学进展,2012,30(2):125-158. |
| 作者姓名： | 吴宁 李燕 沈呈彩 林隽 |
| 作者单位： | 1. 云南师范大学旅游地理学院,昆明,650031 2. 中国科学院云南天文台,昆明650011;中国科学院研究生院,北京100039 3. 中国科学院云南天文台,昆明650011;中国科学院研究生院,北京100039;Harvard-Smithsonian Center for Astrophysics, Cambridge 20138, USA 4. 中国科学院云南天文台,昆明650011; Harvard-Smithsonian Center for Astrophysics, Cambridge 20138, USA |
| 基金项目： | 国家自然科学基金，973计划，中国科学院方向性创新项目，云南省引进海外高层次人才项目，云南省自然科学基金，美国NASA项目 |
| 摘 要： | 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。 |
| 关 键 词： | 耀斑 日冕物质抛射 MHD理论和模型 等离子体不稳定性 磁重联 电流片 |
Large-Scale Reconnecting Current Sheets in Solar Eruptions: Theories and Observations |
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| WU Ning , LI Yan , SHEN Cheng-cai , LIN Jun.Large-Scale Reconnecting Current Sheets in Solar Eruptions: Theories and Observations[J].Progress In Astronomy,2012,30(2):125-158. | |
| Authors: | WU Ning LI Yan SHEN Cheng-cai LIN Jun |
| Institution: | (2,4) (1.School of Tourism and Geography,Yunnan Normal University,Kunming 650031,China;2.Yunnan Astronomical Observatory.Chinese Academy of Sciences,Kunming 650011,China;3.Graduate University of Chinese Academy of Sciences.Beijing 100039;4.Harvard-Smithsonian Center for Astrophysics, Cambridge 02138,USA) |
| Abstract: | 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-1,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 in major eruptions.Plasma blobs flowing with the reconnection outflow inside the current sheet sunward and anti-sunward were observed subsequently,indicating that magnetic reconnection 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 km! 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 consequent 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 inside 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. |
| Keywords: | flares CMEs MHD theories and models plasma instabilities magnetic reconnection current sheet |
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