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太阳爆发的射电和极紫外观测研究
中文摘要

太阳爆发活动是太阳大气中磁场能量的剧烈释放过程,主要包括耀斑和日冕物质抛射(Coronal Mass Ejection:CME)。耀斑是发生在太阳表面的宽谱瞬时增亮现象。CME是从太阳日冕向外抛射的大团磁化等离子体物质,是最大尺度的太阳爆发现象。耀斑和CME经常一起发生,但二者之间并没有必然的因果关系,目前普遍认为它们是同一物理过程在不同方面的表现。它们会产生或增强从射电、白光、紫外、极紫外直至X射线、γ射线的全波段辐射,加速太阳高能粒子,发射向行星际空间传播的磁化等离子体云,以及产生可作为重要粒子加速场所的磁流体力学激波等。当这些异常的扰动现象传播到地球时,会导致空间灾害性天气的发生,威胁人类各种空间设施与相关活动。 本论文中我们主要基于太阳爆发在射电和极紫外两个波段的观测数据,对太阳爆发的表现进行了研究。在第二章我们考察了CME与日冕射线状结构的相互作用对太阳II型射电暴产生的重要作用。在第三章,我们分析了II型暴频带分裂的可能成因。在第四章,我们通过多温度波段高分辨率的全视角多卫星数据对一个活动区间环的形成过程进行了细致分析。 许多研究表明CME与其邻近日冕密度结构的相互作用对太阳II型射电暴的产生有重要作用,但以往的工作都主要通过对射电频谱数据进行分析而得到这个结论,没有使用射电成像数据。在本文中,我们利用弓激波模型以及STEREO/EUVI和SOHO/EIT多视角极紫外成像数据,对被认为是产生2010年3月18日太阳II型射电暴的激波(EUV波)前沿进行了三维重构,得到了整个激波面在三维空间中的位置信息。同时结合从Nancay射电成像数据中得到的射电源区在天空平面上的二维投影,得到了射电源区的三维空间信息。我们发现,这个II型暴源区刚好落在CME(CME)与其邻近日冕射线状结构的交界处,这为II型暴源区位于CME和日冕结构的相互作用区内提供了观测依据。 我们发现并定义了频带分裂II型射电暴的频谱时移现象。通过对事件遍历找到的2005年5月31日频带分裂太阳II型射电暴的研究,我们发现它分裂的两支并没有严格同时出现和变化,而是高频支比低频支提前出现几秒。这个发现为研究II型射电暴频带分裂的物理成因提供了重要的线索和约束,并对上下游理论提出了很大的挑战。这是因为,在上下游理论中,如果在激波传播路径上有可能导致谱形变化的密度结构存在,则低频支源区所在的激波上游会先扫过这个密度结构,然后是高频支源区所在的激波下游扫过。那么这些谱型变化会首先在低频支出现,然后再出现在高频支,这正好与我们的观测结果相反。这表明,被很多人用作理论假设的上下游解释和有可能是错误的。当然在后面的工作中,我们需要更多高时间分辨率的结果来确认这个结论。 为进一步寻找和总结II型暴频带分裂事件的观测特征,我们通过遍历RSTN (Radio Solar Telescope Network,operated by the US Air Force)动态频谱数据找到了18个具有良好的频带分裂特征的太阳II型射电暴事件。我们发现这些取自不同太阳活动周、日冕参数相差很大、相关CME速度相差几倍的不同事件,竟拥有相似的分裂特征,以及相对固定的上下两支之间的频率比γ。在这些事件中,有80%的数据的γ分布在1.15到1.25之问。如果仅考虑数据质量更好、测量误差更小的一组,则95%的数据的γ落在这个区间内。而且,无论是事件之间还是单个事件的整个生命周期中,γ都基本保持不变。另外,我们发现,推导出的源区代理速度和γ之间基本上没有相关性,而这在上下游理论中应该表现为正相关的。最后,我们重新分析了Vršnak et al.(2002)支持上下游理论的证据,发现其引入过多自由参数,且控制因素并不是分裂带宽,其所用数据几倍的不确定性也不足以支持其结论。因此,我们的这些观测结果和上一个工作一起挑战了上下游理论的正确性。 活动区间环(Interconnection Loops;ILs)是连接两个不同活动区的大尺度磁场结构,是CME的一个重要来源。它主要分布于太阳赤道附近,与它关联的CME爆发传播在黄道平面上,对地球空间环境可产生重要影响。且因为ILs具有径向磁场分量,故常被认为是太阳发电机过程中的有效组成部分。一般认为活动区间环是通过高日冕(>150-200角秒)中的磁重联而形成的。目前已有几种图景被提出来解释它们在软X射线波段中的增亮过程,例如重联引起的色球蒸发过程导致致密的热等离子体充填到冕环中点亮冕环,足点磁场增大引起阿尔芬波在冕环中的耗散增强从而加热冕环等。然而,关于导致活动区间环形成的重联的细节研究尚未被报告过,新形成冕环如何增亮的机制问题也仍未解决。在本工作中我们基于在太阳背面的STEREO-A,以及正面的SDO和Hinode的观测数据,我们追踪了活动区间环形成的整个过程。我们发现,此活动区间环是通过高日冕处的磁场重联形成的,这个结论与先前其他研究者的结果一致,同时我们明确指出了在以前的研究中很大程度上被忽略的一点,即冕环在软X射线和极紫外波段的增亮现象是重联点或附近重联的直接加热所导致的,而非由色球蒸发过程所致。 关键字:日冕;CME;激波;太阳射电辐射;活动区间环;磁场重联

英文摘要

Solar eruptions are violent energy release phenomena in the solar atmosphere. Flares and Coronal mass ejections (CMEs) are the most important ones. Flares are sudden brightness enhancement in broad wavelengths, usually observed near the solar surface. CMEs are a large amount of magnetized plasma ejected from the solar corona to the interplanetary space, which are the largest-scale eruptive phenomena in the solar system. Flares and CMEs are normally closely related, but without certain causal association. They are different aspects of the energy-release process in the coronal. They may produce or enhance electromagnetic radiation almost in all wavelengths, accelerate solar energetic particles, generate shock waves and eject magnetized plasma to the interplanetary space. When those anomalous perturbations propagate towards the Earth, they may cause hazardous space weather events and affect our space-based instruments and relevant activities. In this thesis, we conduct observational studies of solar eruptions mainly using the data in radio and EUV wavelengths. In chapter 2, we demonstrate the importance of interactions between CMEs and coronal structures (like rays or streamers) to metric type II radio bursts. In chapter 3, we study the physical mechanisms for band-splits in type II radio bursts. In chapter 4, we present observations of the formation of interconnecting loops and relevant brightening in soft X-rays. Simultaneous radio and extreme ultraviolet (EUV)white-light imaging data are examined for a solar type II radio burst occurring on 2010 March 18 to deduce its source location. Using a bow-shock model, we reconstruct the three-dimensional EUV wave front (presumably the type-II-emitting shock) based on the imaging data of the two Solar TErrestrial RElations Observatory spacecraft. It is then combined with the Nan, cay radio imaging data to infer the three-dimensional position of the type II source. It is found that the type II source coincides with the interface between the coronal mass ejection (CME) EUV wave front and a nearby coronal ray structure, providing evidence that the type II emission is physically related to the CME-ray interaction. This result, consistent with those of previous studies, is based on simultaneous radio and EUV imaging data for the first time. In many type II solar radio bursts, the fundamental and/or the harmonic branches of the bursts can split into two almost parallel bands with similar spectral shapes and frequency drifts. However, the mechanisms accounting for this intriguing phenomenon remain elusive. In this study, we report a special band-splitting type II event in which spectral features appear systematically earlier on the upper band (with higher frequencies) than on the lower band (with lower frequencies) by several seconds. Furthermore, the emissions carried by the splitting band are moderately polarized with the left-hand polarized signals stronger than the right-hand ones. The polarization degree varies in a range of -0.3 to -0.6. These novel observational findings provide important constraints on the underlying physical mechanisms of band-splitting of type II radio bursts. The band split of solar type II radio bursts, discovered several decades ago, is a fascinating phenomenon, with the type II lanes exhibiting two almost parallel sub-bands with similar morphology. The underlying split mechanism remains elusive. One popular interpretation is that the splitting bands are emitted from the shock upstream and downstream, respectively, with their frequency ratio (γ) determined by the shock compression ratio. This interpretation has been taken as the physical basis of many published references. Here we report on an observational analysis of type II events with a nice split selected from ground-based RSTN data from 2001 to 2014, in the metric-decametric wavelength. We investigate the temporal variation and distribution of γ, and conduct correlation analyses on the deduced spectral values. It is found that γ varies in a very narrow range with >80% of γ (one-minute averaged data) being between 1.15 and 1.25. For some well-observed and long-lasting events, γ does not show a systematic variation trend within observational uncertainties, from the onset to the termination of the splits. In addition, the parameters representing the propagation speed of the radio source (presumably the coronal shock) show a very weak or basically no correlation with γ. We suggest that these results do not favor the upstream-downstream scenario of band splits. Coronal loops interconnecting two active regions (called as Interconnecting Loops: ILs) are prominent large-scale structures in the solar atmosphere. They are involved with significant amount of magnetic flux, therefore are considered to be an important element of the solar dynamo process. Earlier observations show that eruptions of ILs are important sources of CMEs. It is generally believed that they are formed through magnetic reconnection in the high corona (>150- 200 arcsecs), while several scenarios have been proposed to explain their brightening in soft Xrays (SXRs), such as the reconnection-induced chromospheric evaporation process which leads to significant heating of dense plasmas, and enhanced heating by dissipation of Alfve'n waves in response to magnetic field increase at loop footpoints. The details of the reconnection process leading to IL formation have not been fully explored and the associated energy release in corona still remains unresolved. Here we report the complete formation process of a set of ILs connecting two nearly formed active regions, with observations by STEREO-A on the far side of the Sun and SDO and Hinode on the Earth-directed side. We conclude that ILs are formed by very-gradual reconnection high in the corona, in line with earlier postulations. Furthermore, we suggest that ILs get brightened in both SXRs and EUVs through direct heating induced by the very-gradual magnetic reconnection in the corona, a process that has been largely overlooked in earlier studies. Key words: corona, coronal mass ejections (CMEs), shock waves, radio radiation, Interconnecting Loops, magnetic reconnection

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