基于有限光谱信息的视频磁像仪反演方法及应用研究

The calibration of magnetic fields, which is the critical issue in observational solar physics, has both important scientific and application significance. The calibrated data can be used not only for solar physics research, but also for the prediction of space environment. With the development of the instrument for the measurement of solar magnetic fields and the polarized radiative transfer theory, the previous calibration methods, especially for the filter-based magnetograph, cannot overcome some shortcomings, such as magnetic saturation and Faraday rotation effects. The reason is that they employ the linear calibration method under the weak-field assumption. In order to overcome the above shortcomings, there is a new method using limited spectral information on the profile of a spectral line, which is adopted by SDO/HMI. In this project, we attempt to calibrate the photospheric and low chromospheric magnetic fields with the existing filter-based magnetograph and the in-development two-dimensional real-time spectrograph installed at Huairou Solar Observing Station, National Astronomical Observatories, Chinese Academy of Sciences. However, being different from the previous methods, the limited full Stokes spectral information observed with the photospheric and low chromospheric lines, is fitted using analytical solution of the polarized radiative transfer equation with the nonlinear least-square fitting method. Based on the relative accurate photospheric and low chromospheric magnetic fields, we try to calculate both the transverse and longitudinal components of the vector electric currents, other than only its longitudinal component derived from the photospheric magnetic field vector.

磁场测量定标是实测太阳物理的基础性工作，定标后的磁场数据既可以开展太阳物理研究，也可以为空间环境预报提供数据支持，具有重要的科学意义和应用价值。随着太阳磁场测量设备和偏振辐射转移理论的不断发展，已有的磁场测量定标工作也存在一些问题和不足。特别是对于滤光器型磁场测量设备，传统的定标方法多采用弱场近似假设下的线性定标，存在磁饱和效应和法拉第旋转等问题。为克服上述不足，近年国际上发展出一套利用有限光谱信息反演矢量磁场的方法，如SDO/HMI采用的定标方法。本项目基于怀柔太阳观测基地现有的滤光器型磁像仪，瞄准在建的、同时成像成谱的两维实时光谱仪，利用有限的偏振光谱信息和偏振辐射转移方程的解析解，采用非线性最小二乘拟合方法开展光球和低色球谱线的磁场测量定标。在相对准确的两个层次的矢量磁场基础上，我们尝试计算矢量电流，突破现有光球矢量磁场仅能计算纵向电流的局限。

磁场测量定标是实测太阳物理的基础性工作，定标后的磁场数据既可以开展太阳物理研究，也可以为空间环境预报提供数据支持，具有重要的科学意义和应用价值。随着太阳磁场测量设备和偏振辐射转移理论的不断发展，已有的磁场测量定标工作也存在一些问题和不足。特别是对于滤光器型磁场测量设备，传统的定标方法多采用弱场近似假设下的线性定标，存在磁饱和效应和法拉第旋转等问题。为克服上述不足，本项目开展了基于怀柔太阳观测基地现有的滤光器型磁像仪，瞄准在建的、同时成像成谱的两维实时光谱仪，利用有限的偏振光谱信息和偏振辐射转移方程的解析解，采用非线性最小二乘拟合方法开展光球和低色球谱线的磁场测量定标。首先我们发展了一系列针对局部像和全日面的偏振成像光谱的定标方法，可以有效提高偏振成像光谱的测量精度；其次，我们发展了光球和低色球的磁场定标方法，不仅仅基于解析解或扩展的解析解反演矢量磁场，而且尝试了基于机器学习的方法进行磁场定标，在解决磁像仪单点观测存在的磁饱和和法拉第旋转等问题上表现良好，提高了单点观测磁场定标的精度。本项目积累的方法可以用于我国首颗太阳观测卫星——先进天基太阳天文台ASOS-卫星全日面矢量磁像仪FMG载荷的偏振成像光谱数据和磁场的定标，具有重要的应用价值。