The Fiber Array Solar Optical Telescope has made key progress

Through the processing and analysis of the observation data obtained by the optical fiber array solar optical telescope (FASOT) principle prototype during the total solar eclipse in Gabon in 2013, the FASOT group of the Yunnan Astronomical Observatory of the Chinese Academy of Sciences has made two key advances in science and technology in the measurement of weakly polarized signals, laying a solid foundation for the first and second generation FASOT under development and the future large-scale coronagraph COMPASS.

The success of routinely obtaining accurate information about the three-dimensional structure of magnetic field vectors in the solar atmosphere largely depends on the instrument’s ability to perceive (sensitivity) and accurately measure (accuracy) the weakly polarized signal generated by a weak magnetic field. Therefore, the sensitive and accurate detection of weakly polarized signals has become a key technology for FASOT.

French astronomer Donati et al. pioneered polarization optical switch technology in 1990 (Astronomy and Astrophysics, 1990, 232, L1) to reduce polarization measurement noise. This technique minimizes the factors affecting the sensitivity and accuracy of polarization measurements by swapping the polarization states of the dual beams emitted by the polarization analyzer. Qu et al. proposed a simplified polarized optical open light (RPOS) technique in 2017 (Solar Physics, 2017, 292:37). It no longer needs to perform double-beam polarization state exchange in each polarization modulation state during conventional polarization observation, and only needs to carry out a set of polarization optical switch mode measurements that can accurately transmit extinction information such as each optical path of the telescope and the degree of opsis at this time, and cross-compare it with conventional polarization observation data to improve the sensitivity and accuracy of polarization measurement. This also identifies new observation modes as well as simplified polarization analyzer structures. In a December 1 paper titled “The Complexity of the Sun’s Upper Atmosphere Revealed by Eclipse Polarization Spectroscopy” (the Astrophysical Journal, 2022, 940:150), the researchers tested the accuracy of the results produced by five methods to achieve RPOS. This technology not only overcomes the three major shortcomings of traditional polarization optical switches, but also improves the sensitivity and efficiency of measurement.

Through the 2013 Gabonese total solar eclipse observation, the FASOT principle prototype obtained for the first time in the world simultaneous observations of radiation intensity I and normalized linear polarization spectrum Q/I of corona, transition region and chromosphere emission lines in the 516-532 nm band in the same field of view (see attached figure). Applying the above technology, the distribution characteristics of the spatial and dispersion directions of these spectral lines I and Q/I formed at different heights were analyzed, and it was found that the rotation rate of the polarization plane of different spectral lines was inconsistent, and the polarization magnitude changes of more than one magnitude would occur at different spatial points, and the polarization of coronal forbidden lines and other spectral lines was very different, and the Q/I profile showed diversity. After explaining these phenomena accordingly, it was concluded that high spectral resolution and simultaneous full-Stokes spectral measurements of two or more spectral lines became essential features of coronal magnetic field vector measuring instruments such as COMPASS.

Figure 1.Spectral line intensity and normalized line polarization intensity change with height

This work is supported by the National Major Scientific Research Instrument Development Project of the National Foundation of China 11527804 and the key project U1931206 of the National Foundation of China and the Astronomy Joint Fund of the Chinese Academy of Sciences. (Source: Yunnan Astronomical Observatory, Chinese Academy of Sciences)

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