冕羽结构光谱学诊断及近地太阳风性质与源区关系的研究

【作者】 付辉；

【导师】 夏利东； 李波；

【作者基本信息】 山东大学 ， 理论物理， 2014， 博士

【摘要】 近地空间环境主要受到太阳活动的影响,其中太阳风是太阳施加影响于近地空间的一个重要媒介,自从太阳风在1962年被发现以来,太阳风的起源,太阳风源区随太阳活动的演化,以及太阳风性质与源区关系的研究一直是太阳物理和空间天气学研究的一个基本课题。本文主要在日冕结构的光谱学诊断和近地太阳风实测分析两个紧密相关的方向开展工作。众所周知,冕洞是太阳风的一个重要源区,而冕羽是扎根在日冕底部一直延伸到高日冕的射线状结构,它是冕洞内最显著的特征结构之一。但是冕羽结构对太阳风的贡献近几十年来一直是有争议的问题。我们首先利用光谱学诊断方法结合日冕紫外成像观测,研究了冕羽结构中的出流,由此讨论了冕羽对太阳风的物质供应问题。进而利用日冕磁场模型作为中介,将太阳风实地探测数据与日面源区性质联系起来,按照源区类型对太阳风进行分类,建立了近地太阳风与其源区在近一个太阳活动周的宏观图像。过去的几十年来,冕羽对太阳风的贡献一直是一个有争议的问题,回答这个问题的重点是获取冕羽内部的出流信息。EIS/Hinode包含很多百万度以上的日冕谱线,使用该仪器我们首次对长时间稳定存在的日面冕羽结构的出流速度进行了测量,发现冕羽结构内部存在稳定的蓝移。更进一步的,我们获取了冕羽内部出流速度随高度的演化信息,在1.02 R☉处为10 km s-1,1.03 R☉处为15kms-1,1.05 R☉处可以达到25kms-1。冕羽结构内的出流速度随着高度有着显著的加速,这一结果对冕羽模型是一个重要的约束。此外,通过分析冕羽结构的密度,对冕羽结构内等离子体出流的通量进行了评估,发现冕羽结构可能是太阳风的一个重要源区。借助于势场源表面磁场模型(PFSS model),我们追踪了1999-2008年ACE卫星观测的太阳风在低日冕高度的源区位置,结合极紫外全日面图像将源区分为冕洞区、活动区和宁静区三类。这使我们可以获取三类太阳风所占比例,以及各类太阳风速度和O7+/O6+随太阳活动的演化。在太阳活动高年,近地太阳风主要来自冕洞区和活动区,并且两者所占比例相当,此时来自宁静区的太阳风可以忽略。在下降年,大部分近地太阳风起源于冕洞区,同时起源于活动区的太阳风逐渐减少,起源于宁静区的太阳风增加。起源于宁静区的太阳风在活动低年最多,此时近地太阳风主要来自宁静区和冕洞区。随着太阳活动降低,所有三类太阳风中表征源区温度的07+/O6+也逐渐降低。另一方面,在同一活动阶段,活动区太阳风速度最低,温度最高,冕洞区太阳风处于另一个极端,而宁静区太阳风性质处于两者之间。我们还对速度低于500 km s-1的低速太阳风进行了分析,发现不同源区的低速太阳风性质的差异和随太阳活动的演化与所有速度范围的太阳风几乎一致。唯一不同的是,在太阳活动低年,起源于宁静区的低速太阳风比例更高,几乎一半以上的近地低速太阳风都来自宁静区。上述结果表明,使用速度和电荷态的联合参数来对太阳风进行分类比单单使用某一项参数要合理。另外不同源区类型太阳风比例的统计结果还表明,宁静区也可能是太阳风的重要源区之一。更多还原

【Abstract】 The solar wind is an important medium through which the Sun can im-pact the Earth and influence the near-Earth space environment. Since the solar wind was first measured in 1962, the origin of the solar winds, the properties of their source regions, and the way how they connect with their source regions are important both as a fundamental issue in solar physics and from the space environment perspective.In the past decades, various efforts have been made to study the source re-gions of the solar wind. Since a long time ago, coronal holes have been believed to be the main source region of the fast solar wind. However, the plasma properties prevailing in coronal holes have not yet fully understood because coronal holes are highly dynamic and structured. For example, plumes are most conspicuous structures observed in coronal holes. Therefore, the contribution of plumes to the solar wind become an important issue. On the other hand, the origin of the slow solar wind is unclear in details. Statistical studies with both in situ observations of solar winds and remote sensing observations of their source regions are needed since the properties of the solar wind are closely related to the source regions. In this thesis, the properties, especially outflow velocities of plumes will be an-alyzed with the spectroscopic observations of EIS/Hinode, and the contribution of plumes to the solar wind will be also evaluated. Furthermore, we identify the coronal sources of the solar winds sampled by the ACE spacecraft during 1999-2008, and examine the in situ solar wind properties as a function of wind sources.The contribution of plumes to the solar wind has been subject to hot debate in the past decades. Evidently, such a debate requires measurements of the electron density and flow velocity in plumes. In this thesis, we measure the outflow velocity at coronal heights in several on-disk long-duration plumes, which are located in coronal holes and show significant blue shifts throughout the entire observational period. Furthermore, we provide an outflow velocity profile along the plumes, finding that the velocity corrected for the line-of-sight effect can reach 10 kms-1 at 1.02 R☉,15 kms-1 at 1.03 R☉, and 25 kms-1 at 1.05 R☉ The clear signature of steady acceleration, combined with the fact that there is no significant blue shift at the base of plumes, provides an important constraint on plume models. At the height of 1.03 R☉, EIS also deduced a density of 1.3×108 cm"3, resulting in a proton flux of about 4.2×109 cm-2s-1 scaled to 1AU, which is an order of magnitude higher than the proton input to a typical solar wind if a radial expansion is assumed. This suggests that, coronal hole plumes may be an important source of the solar wind.We identify the coronal sources of the solar winds sampled by the ACE s-pacecraft during 1999-2008, and examine the in situ solar wind properties as a function of wind sources. The standard two-step mapping technique is adopted to establish the photospheric footpoints of the magnetic flux tubes along which the ACE winds flow. The footpoints are then placed in the context of EIT 284A images, allowing us to categorize the sources into three groups:coronal holes (CHs), active regions (ARs), and the quiet Sun (QS). We find that during the maximum phase, CHs and ARs are equally important contributors to the ACE solar winds, with the contribution from the QS being negligible. During the declining phase, the majority of the ACE winds originate from CHs, whose con-tribution maximizes in 2003. From 2003 onward, the QS contribution increases with decreasing solar activity, and maximizes in the minimum phase when CHs and the QS appear to be the primary suppliers of the ACE winds. With decreas-ing activity, the winds from all sources tend to become cooler, as represented by the increasingly low O7+/O6+ratios. On the other hand, during each activi-ty phase, the AR winds tend to be the slowest and associated with the highest O7+/O6+ratios, and the CH winds correspond to the other extreme, with the QS winds lying in between. Applying the same analysis method to the slow winds only, here defined as the winds with speeds lower than 500 km/s, we find basi-cally the same overall behavior, as far as the contributions of individual groups of sources are concerned. The exception is that, during the minimum phase, the QS may be the important contributor to the slow wind sampled by ACE. The results also suggest that a criterion combining the speed and charge state values may better separate winds from different sources than individual parameters.更多还原