黑子磁场的Stokes光谱反演技术

对太阳大气磁场的可靠测量有助于人们更好地理解太阳活动区内外的许多活动现象,如耀斑的触发和能量释放过程、黑子的形态和黑子大气的平衡、日珥的形成等.由于原子在磁场中的一些能级会产生分裂(Zeeman效应),使对应这些能级的谱线分裂成若干个具有不同偏振特性的分量,因此目前对黑子磁场的测量主要是通过偏振光,即Stokes参量I、Q、U、V的观测来实现的.该文主要介绍近30年来太阳黑子光谱反演的方法以及所取得的成就;同时也对光谱反演和滤光器型的望远镜矢量磁场的测量进行了简单的比较.     

太阳光谱望远镜的Stokes光谱测量

中国科学院云南天文台"太阳Stokes光谱观测与理论研究”团组自进入国家天文观测中心以来,经过一年多的努力,对50cm太阳光谱望远镜原偏振测量部分进行了彻底改造,由原来的D.C.调制改为A.C.调制,且增加了偏振测量校正系统.其机械,电控设计和加工于2000年2月完成,3月完成了光学调试,4月上旬完成了偏振解调和图像处理的软件编制.自2000年4月下旬以来,对23周峰年出现的部分活动区进行了较为成功的斯托克斯光谱测量,其偏振测量精度可达2×10-3,已接近国际水平,为该团组开展的课题研究打下了良好的基础.以此望远镜与怀柔观测基地太阳磁场望远镜相配合,将使我国太阳矢量磁场和速度场的研究更上一个台阶.给出了偏振测量的具体方法和部分测量结果.尽管取得了以上的成绩,但在近期内将对该偏振测量方式作进一步改进,用双光束分析代替单光束分析,使偏振测量精度提高到5×10-4,从而使之不仅能在活动区而且也能在宁静区得到矢量磁场的信息,不仅如此,还可对太阳第二光谱进行测量,提高我们依托该望远镜进行研究的广度和深度.     

云南天文台太阳光谱望远镜加Stokes光电偏振分析器的考虑

云南天文台太阳光谱望远镜是无偏振对称光路系统,经过重新装调望远镜空间分辨率在１”以内,光谱上的空间分辨率在１”～２”,跟踪精度在５ 分钟内１”,以上光学参数达到可以加装Ｓｔｏｋｅｓ 偏振分析器,用于参加２３ 周太阳峰年矢量磁场的光谱轮廓研究     

构建实时三维太阳光谱望远镜的建议

提出了一种新型太阳光谱望远镜的建议，这种望远镜能够同时记录太阳日面观测区域的两维空间的色散（三维光谱），即一系列同步狭窄带通的光谱图像。借助该望远镜我们可以得到高时间分辨率的光谱图（10ms），进而能够做细致的光谱分析。该望远镜由一组子望远镜组成，每个子镜负责记录观测区域的一个事先设计好的透过带，所有透过带覆盖了所研究谱线的整个光谱波段，可以用来诊断不透明的低层大气物质流的三维速度场、重构太阳活动区（即太阳耀斑区）的三维结构。此外，若每个子镜都加栽上偏振仪时，则能够得到精确的矢量磁场，这种矢量磁场能够作为第二代视频磁场测量仪。此望远镜由一组紧密排列的子镜组成，文章分别给出了两种不同排列子镜的方式。描述了用来观测的每个子镜的透过带的样品光学表，并且提出了不同探测器的同时成像技术。最后，我们把该望远镜和ATST（Advanced Technology Solar Telescope）进行了比较。     

云南天文台太阳光谱望远镜偏振分析器检验

介绍了利用云南天文台水平式太阳光谱仪对太阳光谱望远镜偏振分析器进行检验的情况。检验得到的太阳黑子Ｉ、Ｑ、Ｕ、Ｖ四个斯托克斯轮廓与理论轮廓的特征相近,表明该偏振分析器达到了设计要求,用它可以获得太阳黑子的所有斯托克斯参量。     

云南天文台太阳光谱望远镜偏振分析器检验

介绍了利用云南天文台水平式太阳光谱仪对太阳光谱望远镜偏振分析器进行检验的情况。检验得到的太阳黑子Ｉ,Ｑ,Ｕ,Ｖ四个斯托克斯轮廓与理论轮廓的特征相近,表明该偏振分析器达到了设计要求,用它可以获得太阳黑子的所有斯托克斯参量。     

太阳大气中矢量磁场信息的推导

一种能从观测到的斯托克斯轮廓中提取太阳表面矢量磁场信息的方法在本文中提出，它利用斯托克斯轮廓非线心的极值点处相应参量对波长的导数为零这一数学事实，假设表面附近磁场矢量及热力学参量的变化梯度足够小以致所考虑的极值点的波长位置不随深度改变或此变化呆忽则使得偏振辐射围方程组得到极大的简化；再应用数值方法从此简化方程组中解出辐射表面附近的磁场矢量参数。通过拟合理论轮廓表明该法确实可以得到表面近似的磁场矢量     

用斯托克斯轮廓推导太阳矢量磁场和热力学参量的三维结构

一种根据斯托克斯轮廓分析推导太阳矢量磁场和热力学参量的空间三维结构的新方法在本文中提出．在不考虑散射和原子偏振的假设下，该方法由三个具有不同功能的操作分别作用于太阳大气中一层或二层斯托克斯轮廓组成．这三个操作构成一个运行单元．将此单元运用到从表面到光球底层所划分的大气层格点，然后对扫描区所有的点应用同样的程序便可获得太阳上观察区的矢量磁场和热力学参量的空间三维结构．文中给出了相应的流程图和三个操作的详细描述，并用简化了的程序对理论轮廓进行了拟合．结果表明此方法能较满意地导出矢量磁场尤其是磁场强度的三维空间结构，而热力学参量结构的推导还需进一步改进．     

太阳磁场望远镜的进展

由南京天文仪器厂,北京天文台,长春物理研究所和福州物质结构研究所等四单位联合研制的太阳磁场望远镜已在1982年基本完成,目前正在天文仪器厂由北京天文台太阳物理研究室进行试观测。初步结果表明仪器性能良好。 这台望远镜采用拆射系统,物镜口径35厘米,有效视场4′×6′。为了测量在磁场作用下磁敏谱线偏振光的四个Stokes参数,即向量磁场和视向速度场,采用了KD~*P调制器和专门设计的窄带干涉滤光器组成的偏振测量系统。由于不存在歪曲横向磁场的仪器偏振,可以测量向量磁场。仪器备有三种接收系统:照相、光电和视频系统,也就兼有所有三种磁象仪的功能。 1975年,北京天文台太阳物理研究室进行了台址选择的工作,最后选定了北京郊区怀柔水库内的一个小半岛,望远镜将安置在岛端高25米的水泥塔上。观测室土建和园顶正在施工之中。圆顶是特殊设计的,必要时可移出到水泥塔顶北端,与望远镜相距15米。 本文概述了仪器的研制结构和性能,并同美国马歇尔飞行中心的30厘米向量磁象仪作了比较。介绍了进一步提高仪器性能的措施。     

基于双DKDP晶体原理的太阳大气实时偏振探测技术研究

磁场是太阳爆发活动的根本驱动力,高精度实时偏振测量是实现磁场测量的常用方法。常用的波片式测量方法需要进行多次测量,因存在机械旋转结构,易导致仪器抖动,降低测量精度;波片旋转或切入切出也会导致测量时间较长。目前,基于磷酸二氘钾(DKDP)晶体和波片的太阳大气偏振测量周期较长,实时性不高,无法应用于快速变化的太阳活动磁场探测。根据以上背景,提出了一种基于双DKDP晶体原理的太阳大气实时偏振探测技术,采用两片快轴方向不同的DKDP晶体,通过外加不同电压进行快速相位延迟变化,实现对入射太阳光的偏振调制,获取太阳大气偏振的斯托克斯(Stokes)矢量。相较于传统旋转波片与DKDP晶体相结合的方法,本文提出的方法可将单次偏振调制速率从数秒提高到毫秒级,极大地提高了太阳磁场测量的实时性。仿真分析结果表明,本文提出的方法测量精度达到2×10~(-3),单波长点测量时间分辨率提高了20倍以上,可以实现对太阳磁场偏振信息的实时精确测量。     

Stokes polarimetry at the Kodaikanal tower tunnel telescope

A Stokes Polarimeter has been developed using a masked CCD arrangement for the measurement of the vector magnetic field of sunspots. Charge shifting within the CCD is used to record near simultaneous orthogonal polarisation. The testing of the Stokes Polarimeter and the behavior of the integrated system combined with the Kodaikanal tower tunnel telescope will be discussed.     

A quantitative comparison of vector magnetic field measurement and analysis techniques

We make a quantitative comparison between spectral vs filter measurement and analysis techniques for extraction of solar vector magnetic fields from polarimetric data using as a basis the accurately calibrated, high angular resolution Stokes profile data from the Advanced Stokes Polarimeter. It is shown that filter-based measurements deliver qualitative images of the field alignment for sunspots that are visually similar to images derived from the more detailed analysis of the Stokes profiles. However, quantitative comparison with least-squares fits to the full Stokes profiles show that both the strength of the field predicted by the filter-based analysis and its orientation contain substantial errors. These errors are largest for plage regions outside of sunspots, where the field strengths are inferred to be only a fraction of their true values, and errors in the orientation of 40–50° are common. Within sunspots, errors of 20° are commonplace. The greatest source of these errors is the inability of the filter-based measurements to account for the small fill fraction of magnetic fields or, equivalently, scattered light in the instrument, which reduce the degree of polarization. The uncertainties of the full profile fitting methods are also discussed, along with the errors introduced by coarser wavelength sampling of the observed Stokes profiles. The least-squares fitting procedure operates best when the profiles are sampled at least as frequently as one Doppler width of the line.On leave from the Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The Haleakala Stokes polarimeter

A versatile Stokes polarimeter for solar observations has been developed at the University of Hawaii. Recent improvements to the instrument include a high-resolution echelle spectrometer coupled to the telescope by optical fibers, and 128-element diode array detectors. The on-axis design of the telescope and polarimeter limit instrumental polarization to 10^–4, and the spectrometer detector combination provides spectral resolving power of 160000 for any wavelength between 4000 and 11000 Å. This paper describes the Haleakala polarimeter and in particular the spectrometer with its fiber-optic coupling. Examples of Stokes line profiles observed in a sunspot are presented, together with derived vector magnetic field maps.     

The determination of vector magnetic fields in prominences from the observations of the Stokes profiles in the D3 line of helium

A method is presented to measure the magnetic field vector in prominences by means of the polarimetric observations in the D₃ line of He obtained with the High Altitude Observatory Stokes polarimeter. The characteristics of the observed Stokes profiles are discussed. The theory of the Hanle effect is reformulated in the representation of the irreducible tensors of the density matrix, and is generalized to derive the circular polarization profiles across the spectral line in terms of the intensity and direction of the prominence magnetic field. The circular polarization profile so deduced can be employed to obtain useful information which adds to that carried by the linear polarization observations. A non-linear least-squares algorithm is proposed to derive the measurement of the magnetic field from the observations, and a consistency check is suggested to test the adequacy of the theoretical model to describe the physics of the He I atomic excitation in prominences.On leave from: Astrophysical Observatory of Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

High-precision magnetic field measurements of Ap and Bp stars

In this paper we describe a new approach for measuring the mean longitudinal magnetic field and net linear polarization of Ap and Bp stars. As was demonstrated by Wade et al., least-squares deconvolution (LSD; Donati et al.) provides a powerful technique for detecting weak Stokes V , Q and U Zeeman signatures in stellar spectral lines. These signatures have the potential to apply strong new constraints to models of stellar magnetic field structure. Here we point out two important uses of LSD Stokes profiles. First, they can provide very precise determinations of the mean longitudinal magnetic field. In particular, this method allows one frequently to obtain 1 σ error bars better than 50 G, and smaller than 20 G in some cases. This method is applicable to both broad- and sharp-lined stars, with both weak and strong magnetic fields, and effectively redefines the quality standard of longitudinal field determinations. Secondly, LSD profiles can in some cases provide a measure of the net linear polarization, a quantity analogous to the broad-band linear polarization recently used to derive detailed magnetic field models for a few stars (e.g. Leroy et al.). In this paper we report new high-precision measurements of the longitudinal fields of 14 magnetic Ap/Bp stars, as well as net linear polarization measurements for four of these stars, derived from LSD profiles.     

Magnetic Fields and Solar Activities

We discuss the study of solar magnetic fields based on the photospheric vector magnetograms of solar active regions which were obtained at Huairou Solar Observing Station near Beijing in the period of 22nd and 23th solar cycles. The measurements of the chromospheric magnetic field and the spatial configuration of the field at the lower solar atmosphere inferred by the distribution of the solar photospheric and chromospheric magnetic field. After the analysis on the formation process of delta configuration in some super active regions based on the photospheric vector magnetogram observations, some results are obtained: (1) The analysis of magnetic writhe of whole active regions cannot be limited in the strong field of sunspots, because the contribution of the fraction of decayed magnetic field is non-negligible. (2) The magnetic model of kink magnetic ropes, proposed to be generated in the subatmosphere, is not consistent with the evolution of large-scale twisted photospheric transverse magnetic field and the relationship with magnetic shear in some delta active regions completely. (3) The proposition is that the large-scale delta active regions are formed from contribution by highly sheared non-potential magnetic flux bundles generated in the subatmosphere. We present some results of a study of the magnetic helicity. We also compare these results with other data sets obtained by magnetographs (or Stokes polarimeters) at different observatories, and analyze the basic chirality of the magnetic field in the solar atmosphere.     

A possible diagnostic method for magnetic field and velocity gradients in sunspots

We present a diagnostic method for detecting magnetic field gradients and velocity gradients in sunspots through the analysis of Stokes parameters profiles in magnetoactive lines. On one hand, the method is based on the concept of response functions for the Stokes profiles introduced by Landi Degl'Innocenti and Landi Degl'Innocenti (1977); on the other hand, it takes advantage of the diagnostic content of the residuals between observed Stokes profiles and best-fit Unno-profiles. The analysis of synthesis profiles obtained for the line 6302.502 FeI forming in the Sunspot Sunspot Model (Avrett, 1981) suggests that the method could actually be promising, at least if the dependence of the magnetic field vector on optical depth is sufficiently simple.