【作者】 王冰；

【导师】 陈耀；

【作者基本信息】 山东大学 ， 等离子体物理， 2019， 博士

【摘要】 日珥、日冕物质抛射、耀斑及其它太阳大气活动现象都与磁场息息相关。获取日冕磁场的三维信息是研究日珥形成、存在和演化等日冕现象的重要手段。由于日冕高温、稀薄和磁场较弱的特点,直接观测日冕磁场比较困难。当前最有效的认识日冕磁场的办法是基于光球磁图的磁场外推方法。各种方法中,最简单的是势场外推,但是势场中不含有电流,因而不能描述具有扭缠结构的非势日冕磁场;此外,还发展了一些非线性无力场外推方法,可以描述包含场向电流、带有一定扭缠特征的磁场结构;考虑磁场与等离子体相互作用的磁流体力学模拟也是一种被尝试的方法。但需要更多计算资源、且需引入多个等离子体热力学参数。为进一步考虑有力磁场位型、实现大尺度全球日冕磁场的快速重构,我们发展了一个以单层矢量磁场数据为底部边界条件的全球日冕有力场外推方法。该方法基于Hu et al.(2008)提出的直角坐标系下的有力场外推方法,采用基于最小耗散法和变分原理推导出的磁场方程。该方程的解是有力场,可分解为三个分量场,包含一个势场和两个线性无力场。从单层矢量磁图出发,利用一个迭代方法可以获得三个分量场的径向底边界,进而可以利用线性无力场和势场外推方法分别获得三个分量场,求和后便可得到总场。为检验本方法,我们以一磁流体静力学解析解作为参考解,从与之相同的底边界条件出发做全球有力场外推。结果表明,至少在1.5个太阳半径内外推解和参考解符合得很好。外推解的归一化和平均矢量误差(En’,Em’)分别达到～97%和95%。进一步比较它们的磁图、力图和局部磁力线分布,表明所得外推解较好地重构了参考解。日珥是太阳大气中低温度高密度的结构,是高度动态的,其中存在不同的物质流动和振动现象。日珥物质的流动与日珥形成和爆发有关,而与振荡有关的观测数据可用来推测等离子体参数和磁场信息。日珥物质是如何在高温日冕中形成、稳定存在和演化的,仍然是太阳物理研究的一个主要课题。日珥经常存在于日冕暗腔下方,常与向上弯曲的弧状horn结构相连。Horn结构被认为是由日珥向外部延伸的磁力线。这种日珥、horn和暗腔结构间的联系和形成机制仍是一个有争议的问题。关于horn结构的形成,一种观点认为是由高温日冕物质冷却形成的,另一种观点则认为horn结构对应于磁绳分界面处的重联电流片。本文开展了一个典型日珥-horn系统的观测研究,提出了第三种horn结构形成机制。在所研究的事件中,日珥顶部的一部分逐渐抬升并与下方日珥主体分离,从而形成了一个非常简单的位型,便于分析horn结构的形成与演化。我们提出:horn结构是中心冷的日珥物质沿磁场向热的日冕物质稀释过程中所形成。Horn结构的形成与逐渐消失代表了一个日珥物质逐渐加热消散的过程。这与之前文献中所提出的horn是由冷凝汇聚形成的观点相反。此外,我们还研究了日珥物质沿horn结构的周期性大幅纵向振荡,发现振荡周期约为150分钟、振幅约30 Mm;发现horn结构的小幅横向振荡,振幅约3 Mm,周期约10-15分钟,有可能是大尺度日珥-horn结构的全局振动所致。我们还发现实际振荡周期明显大于由所谓日珥振荡单摆模型所得到的周期;horn结构不断地从日珥中分离并升入上方日冕之中,直至日珥爆发,这很可能对应于磁通量不断向上输运日冕磁能不断积累的过程。综上,本论文开展了两项工作:一是发展了全球日冕磁场的有力场外推方法,二是开展了一例日珥-horn结构的动力学观测研究。由于大尺度日珥结构很可能对应与日冕中有力场特征最为显著或集中分布的区域。因此,将来拟基于所发展的有力场外推方法,从观测到的光球矢量磁图出发,可以针对大尺度日珥结构区域做有力场外推,以认识日珥有关磁场结构的有力特性。日珥物理流动及horn结构在示踪相应区域磁场特征方面具有独特的价值,因此,将两个方向的研究结合起来,有望加深对日珥磁场结构、日冕有力场特征的理解。更多还原

【Abstract】 The coronal magnetic field is a critical element of solar physics.Yet,it is d-ifficult to directly measure the vector magnetic field in the corona because of less plasma density,weak magnetic field,and high temperature there.At present,the 3D magnetic fields in the corona are generally obtained by extrapolation methods and numerical simulation.Potential field extrapolation is the simplest method developed so far.Yet,current and magnetic helicity in magnetic field have been neglected.Many numerical algorithms have been developed under the framework of none linear force-free field extrapolation(NLFFF),which includes the effect of twisted fields.Yet,the magnetic force is still not taken into account.Magnetohy-drodynamic(MHD)numerical simulation based on photospheric magnetogram can produce self-consistent results with interaction between magnetic field and plasma.But additional plasma parameters and much more computational re-sources are needed.We present a method of forced-field(i.e,non-force-free field:NFFF)ex-trapolation of the global magnetic field in the corona,on the basis of single-layer vector magnetogram,by extending an extrapolation technique of local magnetic field first developed by Hu et al.(2008).The forced coronal magnetic field is described by a system with the minimum dissipation rate(MDR)which is ap-propriate for the corona as a forced and open system.The obtained solution of the magnetic field can be decomposed into three components including one potential field and two linear force-free fields(LFFF).Starting from the given single-layer vector magnetogram,the bottom boundary condition for each com-ponent is determined with an iterative method to achieve a minimum difference of the transverse component between the extrapolated field and the original mag-netogram.The final extrapolated forced field is given by the sum of the three component fields with the obtained bottom boundaries.The method is tested with an analytic magnetohydrostatic(MHS)solution.It is shown that the ex-trapolated forced field is highly consistent with the MHS solution at least from the solar disk to the heliocentric distance of 1.5 solar radii.For instance,the normalized and mean vector errors(En’,Em’)are as high as～97%and 95%,re-spectively.Further comparisons between magnetic strength,force and field line distributions indicate that the MHS solution has been successfully reconstructed.Formation,stability,and evolution of prominences are the main research topics of solar physics.They are highly dynamic structures with flows and os-cillations.The oscillations provide an approach to obtain information about physical conditions of the prominence.Coronal cavity is the region of the no-tably reduced emission observed from solar limb.The structure generally gets involved in magnetic flux rope that shapes the cavity ellipse-like.There are of-ten some bright U-shape structures,called as horns,at the bottom of the cavity.Physical connections among and formation mechanisms of various components of the prominence-horn cavity system remain elusive.In this study,we present observations of such a system,focusing on a sec-tion of the prominence that rises and separates gradually from the main body.This forms a configuration sufficiently simple to yield elues regarding the above issues.It is characterized by embedding horns,oscillations,and a gradual dis-appearance of the separated material.The prominence-horn structure exhibits a large-amplitude longitudinal oscillation with a period of～150 minutes and an amplitude of～30 Mm along the trajectory defined by the concave horn structure.The horns also experience a simultaneous transverse oscillation with a,much smaller amplitude(～3 Mn)and a shorter period(～10-15 minutes),likely representative of a global mode of the large-scale magnetic structure.The gradual disappearance of the structure indicates that the horn,an observational manifestation of the field-aligned transition region separating the cool and dense prominence from the hot and tenuous corona,is formed due to the heating and diluting process of the central prominence mass;most previous studies suggested that it is the opposite process,i.e.,the cooling and condensation of coronal plas-mas,that formed the horn.This study also demonstrates how the prominence transports magnetic flux to the upper corona,a process essential for the gradual build-up of pre-eruption magnetic energy.In summary,the thesis mainly includes two main parts,a novel method of forced extrapolation of the global coronal magnet ic field and an observational study of dynamics of a prominence-horn structure.It is generally suggested that large scale prominent structures are supported by magnetic forces,and promi-nence are intrinsically associated with forced magnetic field in the corona.There-fore,the two topics of this thesis have close association.In future,it is necessary to study the physics of the large-scale prominence structure with the method of forced field extrapolation of coronal magnetic field based on photospheric vector magnetogram.Furthermore,the plasma flow in prominence and horn structure have significant scientific values to investigate the large-scale configurations of magnetic field.Thus,studies combing the prominence dynamics and magnetic extrapolation are important to further our understanding of prominence forma-tion and dynamics as well as the topology of the coronal magnetic field.更多还原