【作者】 杨凯；

【导师】 丁明德；

【作者基本信息】 南京大学 ， 天文学， 2018， 博士

【摘要】 太阳大气中的等离子体与磁场是高度耦合的,磁场在日冕的加热和动力学过程中扮演了至关重要的作用。观测也显示对地球和空间环境具有重要影响的太阳爆发事件都源自日冕的磁活动。因此对日冕磁场的研究是理解太阳活动、日冕加热机制和开展空间天气预报的一个重要基础。随着现代空间和地面仪器的发展,我们可以以准确度和高时空分辨率来测量太阳光球层的磁场。然而,对日冕磁场的直接测量仍然存在着很大的难度,通常的方法是利用光球的观测数据和适当的磁场模型(如无力场模型)来重构出日冕的三维磁场。据此,我们可以对日冕磁场进行详细的分析,结合磁拓扑理论,研究日冕的磁流体动力学过程。在本论文中,我们利用太阳动力学天文台、日出卫星和拉马第高能太阳光谱成像仪获得的多波段观测资料,运用日冕磁场重构技术和磁拓扑分析方法,对磁场在太阳爆发事件和日冕加热机制等方面所起到的关键作用进行了深入的研究。日冕磁场的连接性将日冕划分成不同的磁拓扑区域。磁拓扑区域的边界(如分界层)是一些非理想过程(如磁重联)发生的场所。磁拓扑的边界在形态上通常与一部分紫外波段的辐射特征相符合,例如在耀斑过程中低层辐射展示的耀斑带对应磁拓扑边界在色球的映射。为了研究磁拓扑结构与太阳爆发事件的相关性,我们对发生于2012年10月23日的X级环形耀斑进行了详细的分析。该耀斑在Ca Ⅱ H谱线的辐射中展示出一个准环形的耀斑带,并且在其内外还存在另外两条狭长的耀斑带。极紫外成像观测显示一个热通道结构,说明此耀斑过程中有磁绳结构存在。此外,热通道的两个足点正好对应两个硬X射线源。利用非线性无力场模型,我们对该活动区进行了日冕三维磁场的重构,并证认出三个磁拓扑结构:一个三维磁零点、一个磁绳结构和一个大尺度准分界层结构。我们发现磁零点包含在大尺度准分界层中,而磁绳结构位于磁零点的扇面之下,并且硬X射线辐射起源于磁绳与周边磁场之间的磁重联。以上三个磁拓扑结构的动力学演化与相互作用导致了该耀斑的爆发以及在爆发过程中所展示出的多波段辐射特征。由于大多数爆发性事件的源区都存在高度剪切的磁拱或者磁绳结构,定量研究这种结构的拓扑性质和时间演化对于理解太阳爆发活动是十分重要的。为此,我们跟踪了太阳活动区12017从2014年3月28日至29日共两天的演化,并通过非线性无力场模型重构了该区域的日冕三维磁场。在两天的时间内,该区域内的暗条活动触发了 12个太阳耀斑,包括9个C级耀斑,2个M级耀斑和1个X级耀斑。在日冕磁场中我们证认了一个磁绳结构,并发现它与暗条在空间和形态上是对应的。我们利用准分界层来定义磁绳的边界,提取出了磁绳自身的空间区域以及其内部的磁场分布。对比耀斑前后的磁绳结构,我们发现闭合性的准分界层(磁绳的包络)所勾勒出来的区域在耀斑发生后有明显的减小。我们还计算了磁绳结构的缠绕数和相对磁螺度,并且发现该活动区大多数耀斑的发生都是由扭曲不稳定性引起的。进一步研究表明,不同特征参数对耀斑的响应是不同的,相对来说磁绳缠绕数的敏感性要高于其他参数(如磁螺度)。除了太阳爆发事件,日冕加热是太阳物理中另一个未解决的重要问题,即日冕等离子体是通过何种方式被加热到百万度的高温。目前已经提出了几种加热机制,比较流行的有阿尔芬波加热和磁重联(纳耀斑)加热。二者在理论上都可以提供日冕加热所需要的能量,但是在一般的情况下,二者都缺乏利用观测数据进行反馈的定量模型。我们基于磁重联加热机制发展了一个利用磁场的观测数据进行约束的日冕结构模型。该模型给出了与极紫外波段观测相符合的辐射特征。在理想等离子体(无磁重联)中,磁场与等离子体冻结在一起,磁力线的足点与周围的等离子体以相同的速度运动。但实际上,由于磁场的耗散(磁重联)磁力线足点和等离子体之间的速度有偏差,这个偏差称之为非理想速度,它与磁重联速率成正比。基于上述物理图像,我们首先从观测到的时间序列的磁图推导了非理想速度的分布以及对应的加热函数,然后基于流体静力学平衡假设推导日冕磁环的热力学结构。该模型不依赖于反常电阻率的假设,一个重要参数是重联磁流管横截面的尺度。该尺度参数可通过模型结果与观测的对比,结合高分辨率的观测资料而得到限制,大约是160 km左右。对比模型生成的和实际观测的极紫外图像,我们发现两者不但在定性上(冕环的形态)而且在定量上(极紫外辐射强度的直方分布)都符合得很好。此外,模型给出的微分发射度与多波段观测反演出的微分发射度具有相似的轮廓。由此可见,我们的研究证实了磁重联加热(纳耀斑)机制可以提供日冕加热所需的能量,并且能够维持一个与观测相符合的活动区日冕结构。更多还原

【Abstract】 It is now realized that the plasma in the solar corona is highly coupled with the magnetic field,which plays a key role in the heating and dynamic processes of the solar corona.The present observations and theories also reveal that the eruptive phenomena in the solar corona,which may seriously affect the solar-terrestrial environment,mostly originate from the magnetic field.Therefore,studies on the magnetic field of the solar corona is crucial for understanding the nature of solar activities and forecasting the space weather.With the developments of the modern instruments,both in space and on ground,the vector magnetic field in the solar photosphere can be measured with high preci-sion and high temporal and spatial resolutions.By employing the force-free model,the magnetic field in the solar corona can be reconstructed with the observed magnetogram as the bottom boundary.This makes it possible to study the topological structure and the magnetohydrodynamic processes of the corona based on the observations.In this thesis,we preform topological analyses of the three-dimensional magnetic field in the solar corona with the multi-wavelength observations from Solar Dynamics Observa-tory,Hinode,and Ramaty High Energy Solar Spectroscopic Imager.We focus on the role of magnetic field in the solar eruptive events(e.g.,solar flares and corona mess ejections)and the heating of the coronal plasma.Based on the magnetic structure,the solar corona can be divided into different topological domains.The boundaries(e.g.,separatrices)between different magnetic domains are places where non-ideal processes(e.g.,magnetic reconnection)favor to take place.These topological boundaries would also trace the morphology of the emis-sion features in observations,such as the flare ribbons.To study the magnetic topology and its association with solar eruptive events,we analyze an X-class circular-ribbon flare on 2012 October 23.The flare showed three ribbons in Ca II H emission,with two highly elongated ones inside and outside a quasi-circular one.A hot channel was found in the extreme-ultraviolet(EUV)emissions that infers the existence of a magnetic flux rope(MFR).Two hard X-ray(HXR)sources in the 12-25 keV energy band were located at the footpoints of this hot channel.With the optimization method,we recon-struct the non-linear force-free field of the active region and identify three topological structures:a 3D null-point,a flux rope below the fan of the null-point,and a large-scale quasi-separatrix layers(QSL)induced by the quadrupolar-like magnetic field of the active region.We find that the null-point is embedded within the large-scale QSL.In this case,all three identified topological structures must be considered to explain all the emission features associated with the observed flare.Besides,the HXR sources are regarded as the consequence of the reconnection within or near the border of the flux rope.Since many eruptive events originate from the highly sheared magnetic arcades or MFRs,it is necessary to perform a quantitative assessment of the topology and evolu-tion of an MFR and its relationship with the associated activities.We reconstruct the magnetic field of active region 12017 from 2014 March 28 to 29,where 12 flares were triggered by the intermittent eruptions of a filament(either successful or confined).From the coronal magnetic field,we find an MFR that is co-spatial with the filament.We determine the boundary of the MFR by a closed quasi-separatrix layer(QSL)en-veloping it.Then,the twist number and the magnetic helicity are calculated for the field lines composing the MFR.The results show that the closed QSL structure(envelope of the MFR)gets smaller as a consequence of the flare occurrence.We also find that the flares in our sample are mainly triggered by kink instability.Moreover,the variation of the twist number is more sensitive than that of other parameters to the occurrence of flares.Besides these eruptive events,a long-lasting problem in solar physics is how the plasma is heated to several millions kelvin in the solar corona.Several different mech-anisms have been proposed,including Alfven wave dissipation and magnetic recon-nection(nano-flares).Both of them are capable of providing the required power,in generic circumstances,neither has yet been used in a quantitative model of observa-tions fed by measured inputs.We show that nano-flare is capable of producing an active region corona comparable both quantitatively and qualitatively with extreme-ultraviolet(EUV)observations.In an ideal plasma without magnetic reconnection,field line footpoints should move at the same velocity as the plasma they find them-selves in.In reality,however,there is a discrepancy observed between the footpoint motion and that of the local plasma due to reconnection,which we name as non-ideal motion.Based on this picture,we come up with a new expression for the heating power proportional to the non-ideal velocity,which can be calculated by using a time series of the observed vector magnetograms.Our model is free from the anomalous resistivity assumption and only depends on the length scale of flux elements reconnected in the corona,which could be constrained from observations and found to be around 160 km in our case.The modeled is free from the anomalous resistivity assumption and only column differential emission measure agrees to a reasonable extent with that derived using EUV images from multiple wavelengths.Synthesized EUV images resemble ob-servations both in their loop-dominated appearance and their intensity histograms.In a conclusion,we provide compelling evidence that nano-flares are a viable mechanism for heating the corona.更多还原