【作者】 章敏；

【导师】 夏利东；

【作者基本信息】 中国科学技术大学 ， 空间物理， 2010， 博士

【摘要】 太阳过渡区爆发事件是过渡区重要的小尺度现象之一,普遍认为其产生机制为小尺度快速磁重联,对其系统、全面地研究始于SOHO时代。SOHO/SUMER具有高时空和谱分辨率、宽的谱线覆盖,其观测的光谱数据为探究过渡区小尺度事件提供了有力的光谱学诊断工具。爆发事件及其它过渡区小尺度现象的形成机制以及它们之间的相互联系(例如,与EUV针状体和闪烁物的联系)一直是太阳物理学家讨论的问题。对这些重要现象及其相互关系的研究有利于我们了解过渡区结构,建立清晰准确的过渡区模型。过渡区小尺度事件的研究对日冕加热、太阳风起源和过渡区能量输运等问题的探究也有很大的推动作用。Lyβ谱线是氢原子Lyman系列谱线中重要的一支谱线。由于辐射转移效应,Lyβ谱线与其它过渡区发射谱线的形状差异很大,普遍呈现非高斯形状(中心反转、两翼形成两峰)。Lyβ谱形受到宁静区与冕洞的不透明性和过渡区流场(稳定的流场和瞬时流动)的影响。对Lyβ谱线形状的研究有利于我们了解太阳大气各区域的结构和流动,深入理解谱线的产生和吸收机制。本文以分析紫外谱线资料为主并结合理论分析,对爆发事件及Lyβ谱形进行了详细的研究,主要结果如下：过去对爆发事件的研究主要基于日面宁静区的观测,冕洞区极少涉及。本文系统研究了极区冕洞内爆发事件及其在低日冕的速度特性。通过极区冕洞内观测的爆发事件的统计分析,得到冕洞内爆发事件的出现率约10-21～10-20cm-2 s-1,爆发事件寿命约2min,与日面爆发事件基本一致。但是,极区冕洞内观测的爆发事件的空间尺度普遍较大(沿狭缝方向尺寸约4″),同时,靠近日面边缘可观测的爆发事件很少,这两个新的发现说明爆发事件的喷流速度主要沿垂直日面方向,所以在视向上不易产生谱线展宽且由于投影效应使其在极区的尺度变大。同时,低温谱线N IV观测的爆发事件在高温谱线Ne VIII相应位置均有响应,表现为蓝移增大或谱线增宽。对爆发事件谱线进行三高斯拟合的结果表明,高温谱线Ne VIII得到的蓝翼平均速度高于低温谱线N IV,表明在这一过程中等离子体既得到加热又得到加速。本文对冕洞区爆发事件的系统分析,是对前人研究的一个重要的补充。日面观测的小尺度事件与太阳边缘观测到的小尺度事件之间的联系一直被探讨,但不够深入。本文通过宁静区和极区冕洞观测到的OⅣ、NⅣ和Ne VIII谱线的速度图,研究了两区域小尺度速度跃变事件的强度、宽度等参数的变化规律。跃变事件的蓝移、红移速度可达每秒几十千米。跃变事件的尺寸、寿命与常见的日面边缘EUV针状体大致相符。速度跃变事件初始时,一般在一些强的蓝移点处产生爆发事件,谱线呈现非高斯形状,寿命约几分钟,有的光谱可见明显的双向喷流,且均能被高温谱线NeⅧ所观测。有的爆发事件产生时谱线强度急剧增加,导致局地亮度突然增强,类似于闪烁物增亮事件。冕洞内可观测到准周期约16分钟的小尺度速度跃变事件,其强度振荡变化可判断出闪烁物事件。因此,我们可以推断,本文在极区冕洞内观测的速度跃变事件与爆发事件、闪烁物和EUV针状体有着紧密的联系。Lyβ谱线形状受到区域大气不透明性与流动的影响。我们统计研究了宁静区、赤道和极区冕洞各区域Lyβ的谱形。通过分析冕洞和宁静区按强度分类得出的Lyβ谱形,我们发现：冕洞区Lyβ谱线强度越大,谱线中心反转越强；反之越弱,最弱的谱线有的无反转。宁静区Lyβ谱线红峰高于蓝峰,赤道冕洞的Lyβ谱线红峰与蓝峰强弱相当,极区冕洞的Lyβ谱线蓝峰却高于红峰。各区域谱线形状随强度渐进性变化。同时对Lyβ谱形与CⅡ和OⅥ谱线的速度关系的统计分析表明,CⅡ谱线的多普勒速度越大,Lyp谱线的不对称性越明显。赤道冕洞的Lyβ谱线的反转中心与OⅥ谱线线心位置有较明显一致性。我们发现宁静区Lyβ谱线的强度仅与OⅥ速度大小相关,随着OⅥ视向速率变小Lyβ谱线的强度也变小。Lyβ谱形的这种复杂变化现象可能说明该谱线包含了丰富的关于太阳色球层和过渡区的动力学过程的信息。太阳动态的过渡区存在各种瞬变现象产生的瞬变流场可能导致Lyβ谱线形状的变化,我们首次研究了过渡区爆发事件喷流对Lyβ谱线形状的影响。通过赤道冕洞和宁静区CⅡ、OⅥ谱线的爆发事件诊断,得出了CⅡ事件和OⅥ事件对应的OⅥ、Lyβ和CⅡ谱线的平均谱形。不论宁静区还是冕洞,爆发事件的Lyβ平均谱线的中心反转强且有明显的两峰。这种趋势在CⅡ事件的Lyβ谱线上更明显。相关性分析表明,爆发事件发生时CⅡ和OⅥ谱线的两翼和Lyβ两峰的辐射强度均有正相关性,且两者的相关性在CⅡ谱线的两翼和OⅥ的红翼较好,但在OⅥ的蓝翼较弱。本文研究证明,瞬变的过渡区流场可以明显改变Lyβ的谱形。因此,在分析Lyβ等赖曼谱线的谱形所反映的物理过程时,既要考虑谱线的源函数、不透明度,也要考虑过渡区和日冕中的流场,包括准稳定的流场和瞬变的流场。本文对于Lyβ谱形研究也将推动Lyα谱线对动态现象响应的研究,其谱形对动态现象(小到过渡区爆发事件,大到耀斑和CMEs)的响应将是研究太阳爆发的一个新思路。更多还原

【Abstract】 Explosive events (EEs) are small-scale dynamic phenomena often observed in the solar transition region (TR). They have been suggested to be a consequence of small-scale fast magnetic reconnections. Since the launch of SOHO spacecraft, the SUMER spectrograph has been widely used to study EEs. With high spatial and spectral resolution, and wide spectral coverage, SUMER was a powerful tool of ultraviolet spectroscopy and has greatly increased our knowledge of EEs and other small-scale TR phenomena. The physical mechanism of these small-scale TR events and their relation with each other has been extensively discussed, but remain unclear. Investigation of EEs and other TR events, such as blinkers and EUV spicules is importance to understand the structures and dynamics prevailing in the TR and to further improve the modeling and theoretical description of this region. Study of these dynamic phenomena is also important to.understand the energy transportation in TR and the process of coronal heating, origin and acceleration of the nascent solar wind. On the other hand, Lyp is one of the most important spectral line of hydrogen Lyman series. Important information on the highly dynamic TR structures may be carried by the profiles of this line. As affected by radiative transfer process, Lyβprofiles are often characterized by non-Gaussian (self-reversed at the centers, two peaks aside). Studies on the sharp of Lyβprofile are helpful to understand the structures and flows in different regions of the solar atmosphere and the formation and absorption mechanism of the line itself. In the thesis, our studies will be focused on EEs and their impacts on the Lyβprofile, based on detailed analysis and interpretation of the data observed by SUMER in the ultraviolet wavelengths. Our major findings are reported as follows.In the past, EEs were mainly studied with the data observed on the disk of quiet-Sun (QS) region, while their properties in coronal holes (CHs) remain unclear. Here, we study EEs and search for their signatures in the lower corona. Our statistical study shows that the EEs have an occurrence rate of about 10-21～10-20 cm-2 s-1 and an average lifetime of around 2min, which are consistent with previous results obtained on the disk in QS regions. However, EEs in polar CHs generally have a larger spatial size (～4 arcsec) on average. We also find that EEs are rarely observed near the limb. These two findings imply that the jet structure of EEs is likely perpendicular to the solar surface, so that they are not easy to form the broad line width and their size also becomes larger due to the line-of-sight effect when observed in the polar CH. Meanwhile, the NeⅧline formed in the high TR/lower corona responds to the EEs observed with the NⅣline formed in middle TR, during which an increased blue-shift or broadening line width is usually present. When fitting all EE profiles with three-Gaussians, we find that the average velocity of the blue jet component deduced from the NeⅧline is higher than that obtained from the NⅣline. This indicates that the plasma has been both accelerated and heated in this process. Our results are an important complement to the pervious studies of EEs on the disk.Although the connection between the disk and limb small-scale events has been mentioned in some literatures, there was no firm conclusion. In this study, we start with the velocity maps deduced by TR lines including the OⅣ, NⅣand NeⅧlines, where structures with velocities rapidly changing between red and blue shift (referred to as "transient velocity events") are often observed. It is interesting to know how the line intensity and width change during these events. We find that the maximum of blue/red-shifted velocities of such events can reach several tens of kilometers per second in both the QS and polar CHs. The velocity, spatial size and time duration are consistent with those of EUV spicules observed above the limb. At the initial stage of the transient velocity events, EEs are often observed at the locations with very strong blue shift. In this case, line profiles of NⅣand OⅣbecome non-Gaussian, which may last for several minutes and can also be seen at the NeⅧline. Sometime, bi-directional jets also present. Some EEs have a sudden enhancement of the intensity, which can obviously be identified as blinker-like events. In the polar CH, some transient velocity events are obviously repetitive with a period of～16 min. Blinkers are also identified in these events according to their intensity oscillation. Therefore, we can infer that the transient velocity events found in the polar CHs are closely connected with EEs, blinkers and EUV spicules.The line shape of Lyβis affected by the opacity and flows in the solar chromosphere and TR. We statistically analyze the Lyβprofile in the QS, equatorial CHs and polar CHs. By analyzing Lyβprofiles classified by its intensity, we find that the Lyβreversal deepens with increasing line intensity in CH regions. The profile becomes nearly Gaussian (without reversals) in cell regions. The red peak of Lyβprofile is stronger than the blue peak in QS regions, becomes basically the same in equatorial CHs, and is obviously weaker than the blue peak in polar CHs. We also find the Lyβprofile of each region varies gradually with its intensity. The statistical analysis of the relationship between the sharpness of Lyβpeaks and the velocity of CⅡI and OⅥline indicates the larger of the Doppler velocity of C IⅡ, the more obvious of the asymmetry. It seems that the wavelength position of Lyβcentral reversal varies with the center position of OⅥ. We also find that the intensity of Lyβin QS regions decreases with decreasing velocity of OⅥ. The complicated variations of Lyp are likely to indicate that this line contains abundant information on dynamic process in the solar chromosphere and TR.The variations of the shape of the Lyβline may be affected by transient flow fields in the dynamic solar TR. For the first time, we search for signatures of EEs in this line. EEs are identified from profiles of the CⅡand OⅥlines, respectively. We compare Lyβprofiles during EEs with those averaged in the entire QS and CH regions. We find that the central part of Lyβprofiles becomes more reversed and the distance of the two peaks becomes larger during EEs, both in the CH and QS region. The average Lyβprofile of the CⅡEEs has an obvious stronger blue peak. During EEs, there is a clear correlation between the increased peak emission of Lyβprofiles and the enhanced wing emission of the CⅡand OⅥlines. The correlation is more pronounced for the Lyβpeaks and CⅡwings, and less significant for the Lyβblue peak and OⅥblue wing. We also find that the Lyβprofiles are more reversed in the CH than in the QS. The finding that transient flows produced by EEs modify the Lyβline profile in QS and CHs implies that one should be careful in the modeling and interpretation of relevant observational data. When the underlying dynamic process of the solar atmosphere is analyzed by using Lyβand other Lyman lines, one should consider not only the line source function and opacity, but also the flow field in the transition region, including both the quasi-steady and transient flows. The research of Lyβline also has an implication to the Lya line in response to such dynamic phenomenon. Furthermore, it would be a new idea to study other dramatic eruption events (e.g., flares and CMEs) of the Sun through investigation of the shape of Lya line.更多还原