基于衰减横波的冕震学工具的可靠性研究

Measuring the magnetic field and density structuring in the solar corona is a key issue in space physics. However, the weak coronal emission combined with the limited spatial resolution of the currently available space-borne instruments makes such measurements difficult. An alternative approach based on the measured damped fast kink waves in coronal loops, the so-called "coronal seismology" approach, is the primary means for deducing them at present. However, the theoretical foundation on which this indirect approach is based is far from realistic. Assessing the uncertainties thereby caused, although crucial, has not been addressed. We will construct fully three-dimensional models of coronal loops, as realistic as possible, and examine the excitation, propagation, trapping and damping of fast kink waves that the loop system supports in a unified and self-consistent manner. We will then use the output from this three-dimensional model to synthesize observables such as the emission intensity and line profiles, which will then be used to deduce the coronal parameters using the standard seismological tools. By comparing the parameters hence deduced with those computed directly from the three-dimensional model, we will be able to evaluate how reliable these seismological tools are. We will trace the temporal evolution of fast kink waves in a realistic coronal loop and pinpoint their damping mechanism. In particular, we will elucidate the relative importance of the primary competing ideas for explaining this damping, namely, resonance absorption, phase mixing, and lateral leakage. The results will enable us to tell the reliability of the seismological tools based on the measurements of the wave periods and damping time/length, as far as the deduction of the coronal magnetic field and density structuring is concerned.

日冕磁场、密度不规则性等关键参数的探究是空间物理学焦点问题之一。由于日冕辐射暗弱及探测设备空间分辨率的限制，它们难以直接测量。基于冕环中衰减快扭曲波观测的"冕震学"工具是提供这些参数的主要间接手段，但其理论基础偏离太阳大气现实甚远，所造成的参数推测的不确定性还极少研究。本项目拟构建日冕磁环的尽可能贴近现实的磁流体动力学模型，对其中快扭曲模行波与驻波的激发、传播、束缚及衰减进行统一而自洽的研究；由此基于光谱学计算工具，合成辐射强度、谱线轮廓等成像及光谱学可观测量，进而利用标准的现有冕震学手段对冕环物理参数加以推测，通过其与"真值"的对比，定量评估现有冕震学推测手段的可靠性。特别地，将追踪"真实"冕环中快扭曲波的演化，探讨波动衰减的物理机制，明确共振吸收、相混合及横向泄露等主要衰减机制的相对重要性，由此明晰波动周期及衰减时间（或长度）作为冕震学工具来诊断日冕磁场和不均匀性的不确定性。

日冕磁场、密度非均匀性等参数对日冕加热机制而言至关重要，但难以直接常规测量。基于冕环中快横波观测的“冕震学”反演是获取这些参数的主要间接手段，但反演工具之理论基础往往偏离现实甚远，由此导致的反演值的不确定性还少有研究。本项目综合利用解析、数值模拟及资料分析手段，开展了如下工作：1)横波衰减机制。全三维磁流体数值模拟研究表明，当环足受到扭曲型扰动后，扭曲模的演化早期（约十个纵向Alfven时间内）集体扰动的衰减以共振吸收为主导机制，之后局域Alfven波非线性驱动的KH涡旋在扰动耗散中起重要作用。对快腊肠模而言，横向泄漏是造成表观衰减的主要原因。2)横波的光谱学表征及其观测判据。对扭曲模的正演结果表明，以Hinode/EIS为代表的现有光谱仪受限于空间分辨率而无法分辨局域Alfven波或KH涡旋。对腊肠驻波的正演研究则表明,其对活动区环典型极紫外谱线参数的调制计算需计入非平衡电离效应，在调制耀斑线展宽时所带来的半周期不规则性可作为腊肠波观测新判据。3)以冷磁流体为出发点、不考虑磁结构内亚分辨率精细结构、只计入磁结构参数横向连续分布这一核心现实特征开展反演时，单以扭曲驻波之周期和衰减时间观测作为输入，磁场强度反演值的不确定性至少在30%，而密度非均匀尺度则无法约束，此不确定性完全来自密度横向分布描述无法由观测给出这一现实困难；单以腊肠驻波相应观测为输入，反演值之不确定性更显著。但如所观测震荡信号同时包含两种波模，则反演值的不确定性可大为降低。计入冕环内气体热压通常不造成额外不确定性，但需将冷磁流体反演所得Alfven速理解为横向快波速。计入冕环内精细结构至少对利用腊肠驻波的反演结果不导致额外不确定性。光谱或成像之正演与冕震学反演结果的相互校验不仅可更好约束反演值，还可甄别非均匀性的描述方案。利用色散横波列之小波谱观测也可达到同样效果。利用研究成果，项目组成员共发表第一或通讯作者论文16篇（含12篇ApJ）。