Research progress in coronal magnetic field measurements
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摘要: 磁场把太阳的各层大气耦合在一起,并主导着其中的各种物理过程,磁场的演化引发包括太阳爆发在内的活动现象,进而影响着地球的空间环境和人类的生存条件. 要理解发生在太阳大气中的各种活动现象,离不开对其磁场的完整认知. 然而目前对太阳高层大气尤其是日冕的磁场测量仍然严重缺失,这大大限制了人类对太阳活动的研究,制约了太阳物理和空间天气学科的发展. 经过几十年的探索,人们提出了几种可能可用于日冕磁场诊断的方法,包括利用日冕红外谱线的偏振观测、借助射电波段的日冕辐射、通过日冕中各类波动的冕震学诊断以及利用日冕极紫外谱线的磁场诱导跃迁原理对日冕磁场进行测量等. 同时,人们也根据这些方法尝试对日冕磁场进行了测量,取得了一些进展. 本文总结了几种主要的日冕磁场测量方法的原理和重要进展,并对未来的相关研究做了展望.Abstract: Different solar atmospheric layers are connected by the magnetic field of the Sun, which is the primary source for various types of solar activity. The magnetic activity of the Sun has a significant impact on the solar-terrestrial environment and human life. Understanding the phenomena occurring in the solar atmosphere relies on a thorough understanding of the solar magnetic field. However, up to now, only the magnetic field at the photospheric level can be measured with precision on a daily basis. No routine measurements have been carried out for the magnetic field in the upper layers of the solar atmosphere, particularly the solar corona. The lack of coronal magnetic field measurements has limited our investigation of many important topics in solar physics research including the driving mechanism of solar eruptions and the heating process of corona. In the past several decades, a number of techniques that may be useful for coronal magnetic field diagnostics have been developed, such as the techniques based on infrared spectro-polarimetry of coronal lines, the diagnosis utilizing coronal radio observations, magneto-seismology using observations of coronal magnetohydrodynamics waves and the measurements based on the magnetic-field-induced transition of coronal extreme ultraviolet spectral lines. Meanwhile, a few attempts have been made to measure the coronal magnetic field based on these techniques. This review summarizes the physical principles and important progress of coronal magnetic field measurements, and discuss future perspectives on related research.
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Key words:
- solar corona /
- magnetic field /
- solar atmosphere
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图 1 塞曼效应的经典描述图像. 从不同方向观测到的偏振特性不同,分别对应着纵向塞曼效应和横向塞曼效应(修改自Lites, 2000)
Figure 1. The classical description of longitudinal and transverse Zeeman effect. The observed polarization states vary with the observer's viewing direction (modified from Lites, 2000)
图 2 观测到的Fe XIII 10747 Å的斯托克斯Q和V轮廓,可以看到斯托克斯V的反对称轮廓(修改自Lin et al., 2000)
Figure 2. The observed profiles of Stokes Q and V from Fe XIII 10747 Å, the anti-symmetric profile of Stokes V was clearly observed (modified from Lin et al., 2000)
图 3 使用EOVSA观测到的回旋同步加速辐射诊断得到的耀斑区域的二维日冕磁场分布的时间演化. (a)~(d)代表不同的时刻(修改自Fleishman et al., 2020)
Figure 3. Temporal evolution of coronal magnetograms in a flare region, as diagnosed through gyrosynchrotron emissions observed with EOVSA (modified from Fleishman et al., 2020)
图 4 对CoMP观测到的扭曲模行波做冕震学诊断得到的日冕磁场的全局性分布图(修改自Yang et al., 2020a)
Figure 4. Global map of coronal magnetic field obtained through magnetoseismology using CoMP observations (modified from Yang et al., 2020a)
图 5 利用前向模拟验证磁场诱导跃迁方法可行性的结果. (a,c)模型中日面上和日轮边缘外(视线方向垂直日面和平行日面)的日冕磁场强度分布. (b,d)根据前向模拟,利用MIT方法诊断得到的磁场强度(修改自Chen et al., 2021)
Figure 5. (a, c) Spatial distributions of coronal magnetic field strength in the model for disk-center and off-limb views, respectively. (b, d) The derived magnetic field strength based on the MIT technique using forward modelling (modified from Chen et al., 2021)
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