YNAO OpenIR  > 太阳物理研究组
Alternative TitleNumerical Experiments on Physical Properties of the Current Sheet in the Solar Two-ribbon Flare
Thesis Advisor林隽 ; John C.Raymond
Degree Grantor中国科学院研究生院(云南天文台)
Place of Conferral北京
Degree Discipline天体物理
Keyword太阳   日冕物质抛射   耀斑   磁重联   电流片   不稳定性   电离   发射线强度
Abstract大尺度的磁重联电流片是太阳爆发过程中的重要结构。在日冕物质抛射(CME)/耀斑理论模型中,它连接着耀斑环并向上延伸到相应的磁通量绳(即CME)。研究大尺度电流片的物理性质有助于理解太阳爆发过程中的快速磁重联过程、不稳定性和湍动过程、以及高能粒子加速机制。本文旨在通过数值实验研究此类电流片的物理性质,分析电流片内的细致结构,计算大尺度电流片的发射线强度并预期电流片的观测特征。 我们通过有限电阻磁流体模拟来分析双带耀斑电流片内的精细结构。我们的计算从处于静力学平衡和热平衡的磁位形开始。在该位形中,磁力线固结在代表光球的底边界上,一个初始的电流片分隔了包含反平行磁场的两部分等离子体区域。在初始扰动的驱动下,磁重联过程在该电流片内缓慢发生,在第一个磁岛出现后迅速加快,并在电流片内产生了许多小尺度结构。这些结构包括沿电流片运动的多个等离子体团和等离子体团间的磁重联X点。我们分析了电流片的演化过程并总结了这些等离子体团沿电流片的运动特征。另外我们使用谱分析方法讨论等离子体团以及它们所包含能量的统计性质,结果表明电流片内磁能和动能能谱的分布都遵循幂律分布。相应的谱指数随系统初始的磁雷诺数而变化,并在高磁雷诺数时$ ({R_{m}}$ ${> 10^5})$趋向于某一常数。磁能能谱变化与电流片内磁岛的动力学行为有关,例如磁岛的生长会导致磁能能谱变陡,而大尺度磁岛离开计算区域则使能谱变平。 为了进一步研究大尺度电流片的观测特征和电流片内部等离子体的物理性质,我们根据Reeves 等人的CME/ 耀斑模型并使用相应的数值模拟结果,考虑电流片内等离子体流的非平衡态电离过程,并预期了该电流片在太阳动力学天文台(SDO)/AIA各波段观测的强度和在SOHO卫星/UVCS观测中的谱线强度,考察了电流片当中等离子体的电离状态和一些几何特征。结果表明电流片底端的等离子体处于亚电离状态而顶端的等离子体处于过电离状态。通过对比非稳态电离过程和平衡电离过程,我们得出在电离平衡假设下,电流片底端的温度会被高估而电流片顶端的温度会被低估。最后我们也讨论了在电流片的观测中,在UVCS谱线分布上可能观测到低发射强度的鞘形区域。
Other AbstractLarge scale current sheets are important structures in solar eruptions. Models of coronal mass ejection (CME)/flare include the large scale current sheet connecting the ejected flux rope to the post-flare loop. Studies of physical properties in current sheets could improve our understanding on rapid magnetic reconnection during a solar eruption, instabilities and turbulence and high-energy particle acceleration taking place inside the current sheet. This thesis focuses on the physical properties, internal structures, and the features of emission lines of current sheets that form during solar eruptions by performing numerical experiments, and giving predictions from CME/flare models to be compared with observations. We perform resistive magnetohydrodynamic simulations to study the internal structure of current sheets that form during solar eruptions. The simulations start with a vertical current sheet in mechanical and thermal equilibria, which separates two regions of the magnetic fields of opposite polarity that are line-tied to the lower boundary representing the photosphere. Reconnection commences gradually due to an initially perturbation to the system, but becomes faster when plasmoids appear and produce small-scale structures inside the current sheet. These structures include magnetic islands or plasma blobs flowing in both directions along the sheet, and X-points between pairs of adjacent islands. We show detailed evolutions of the current sheet and look into dynamical features of magnetic islands or plasmoids. Then we examine the statistical properties of the fine structure and the dependence of the energy spectra on these properties. The spectral profiles of magnetic and kinetic energy inside the current sheet are both of the power law. The corresponding spectral indices are found to vary with the magnetic Reynolds number $R_m$ of the system, but tend to approach to a constant for large ($R_m$ $> 10^5$). The motion and growth of blobs change the spectral index. The growth of new islands causes the power spectrum to steepen, but spectrum becomes shallower when old and large plasmoids leave the computational domain. To understand observational features and physical properties of CME/flare current sheets, we deduced some important results and the corresponding observational consequences from flare/CME models in order to compare with observations, such as those from Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory and Ultraviolet Coronagraph Spectrometer on the Solar and Heliospheric Observatory. We used a simulation of a large-scale CME current sheet previously reported by Reeves et al., and performed time-dependent ionization calculations of the plasma flow in a CME/flare current sheet. The results show differences in the emission line intensities between equilibrium and non-equilibrium ionizations. The current sheet plasma is found under-ionized at low altitude and over-ionized at high altitude. The assumption of ionization equilibrium would lead to a significant underestimate of the temperature low in the current sheet and overestimate at larger heights. By assuming intensities of emission line, we compute the count rates for each of the AIA bands and emission features, and compare the results with observations from UVCS, including a low intensity region around the current sheet corresponding to this model.
Subject Area天文学
Document Type学位论文
Recommended Citation
GB/T 7714
沈呈彩. 关于太阳双带耀斑电流片物理性质的数值实验[D]. 北京. 中国科学院研究生院(云南天文台),2013.
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