Probing Magnetic Fields in the Solar Convection Zone with Time-Distance Helioseismology

以日震學的時距分析法探究太陽對流層內之磁場

梁智超

指導教授 ： 周定一

國立清華大學/理學院/物理學系/博士(2014年)

https://doi.org/10.6843/NTHU.2014.00392

摘要

摘要

大部份太陽活動皆起因於太陽磁場，於是太陽學家對太陽內部的磁場起源便充滿了興趣，尤其是太陽對流層底部。日震學借由研究太陽表面的震波能夠探測太陽內部，而要從震波推斷太陽內部磁場，還需要完整了解波與磁場的交互作用。這篇論文包含了在對流層底部有磁場存在的觀測證據，震波被黑子散射的特徵，還有一個用來描述震波通過黑子時的能量分佈模型，整個工作都使用了日震學中的時距分析法。

首先我們呈現的是在整個太陽對流層裡子午流隨著太陽週期變化的觀測結果，我們用日震方法分析了十五年的衛星資料來研究太陽內部子午流隨時間的變化。我們測量了震波在表面上子午方向(南北向)的任兩點間傳播所花的時間，這可以用來推斷波在這兩點間行進的路徑上介質流動的速度，這其中我們處理了許多系統誤差，特別是來自表面磁場對測量結果的影響。觀測結果顯示了對流層底部在太陽極大期與極小期存在差異，此差異很可能是對流層底部的磁場造成的，以此推估磁場大小至少有兩千高斯左右。

其次我們辨識了當震波波長與黑子大小差不多時，震波被黑子散射的特徵。我們不只測量了散射波相對應於入射波的相位差同時也測量了振幅，在這裡平面波被分離出來作為入射波。在相位差的測量中，一個在散射理論中描述的波前癒合的現象被觀測到了，這是一個必須考慮波本質的現象(near-field region)，而不能只考慮波程差(far-field region)。另一方面，振幅的測量結果顯示了在波通過黑子的中心路徑上振幅減弱了，而在中心路徑兩旁則有增強的現象，這表示波被黑子加快了速度，使入射波被散射到路徑兩旁，這是除了波通過黑子直接被黑子吸收外，另一個造成振幅衰減的原因。

論文最後我們提出了一個模型來解釋黑子中震波振幅衰減的行為，我們把它分成了三個機制，吸收(absorption)、發射減弱(emissivity reduction)、局部抑制(local suppression)。相關係數(cross correlation function)的計算能夠過濾掉不從起始點出發的信號，所以我們利用這個特性將三個機制分離開來，接著我們挑選了一個黑子作為案例研究，計算其相關係數的強度來決定三個機制對振幅衰減的貢獻。由這三個機制推出來的振幅衰減程度與實際觀測結果差異不到百分之七。

關鍵字

日震 ； 黑子

並列摘要

Abstract

The magnetic field is responsible for most of solar activities on the Sun. Subsurface magnetic field is therefore of great interest to the solar physicists, especially the origin of it at the base of the convection zone (BCZ). Helioseismology is capable of exploring the solar interior by studying the helioseismic waves observed on the surface. To infer the magnetic field inside the Sun from solar oscillations, one requires a comprehensive understanding of the interaction between waves and the magnetic field. This dissertation contains the observation of evidence of magnetic field at the BCZ, the characteristics of helioseismic waves scattered by a sunspot, and a model for the energy budget of solar acoustic waves propagating through the active region. Time-distance method in helioseismology is employed throughout this work.

First, we present the observation of the solar-cycle variation of meridional flows in the entire convection zone. Time-distance helioseismology has been applied to SOHO/MDI spacecraft data to study the temporal variation of subsurface meridional flows over 15 years. The time it take for solar acoustic wave to travel between two points on the surface in the meridional plane is measured, which can be used to infer the flow in the vicinity of the ray path that connects two surface locations. A number of systematic errors have been confronted, particularly the contamination from the surface magnetic field. Observations show difference between solar maximum and minimum at the BCZ. They suggest the presence of magnetic fields around the BCZ, and place a lower limit of 2 × 10^3 gauss for the field strength at the BCZ.

Second, we characterized features of the acoustic-wave packet scattered by a sunspot in a regime where the wavelength is comparable to the size of the sunspot. Not only do we measure travel-time shifts but also wave-amplitude perturbations with respect to the quiet Sun. Plane wave packets are sorted out as the incident waves. A finite-wavelength phenomenon known as wavefront healing in scattering theory has been observed in the travel-time measurement. On the other hand, the observation shows on-axis reductions and off-axis enhancements in wave-amplitude measurement. They indicate that the sunspots may introduce a fast wave-speed perturbation and defocus the wave energy in addition to the physical absorption of waves by sunspots.

Last, we propose a model for the energy budget of acoustic waves propagating through a sunspot in terms of the coefficients of absorption, emissivity reduction, and local suppression of the sunspot. Effects of three mechanisms are separated by computing the cross-correlation function (CCF) which excludes the signal that is not emitted from the starting point. We then determine the fraction of three mechanisms contributing to the energy deficit from the magnitude of CCF with a selected sunspot as a case study. The acoustic power derived from the determined coefficients is consistent with that obtained directly from the acoustic-power map within 7%.

並列關鍵字

helioseismology ； sunspot

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