In a study published online in Monthly Notices of the Royal Astronomical Society, LIANG Yu and Prof. QU Zhongquan from Yunnan Observatories of the Chinese Academy of Sciences, and their collaborators, proposed a high-precision coronal image registration method for imaging polarimetry of the coronal red line.

The coronal red-line polarimetry data obtained by PKU’s Coronal Red line Image Telescope (CRIT) during the 2017 total solar eclipse and the Mauna Loa Solar Observatory (MLSO)/KCor data were analyzed. The results showed that the red-line radiation may be polarized in the dark coronal cavity region.

Normally, polarimetry requires multiple modulations of different polarimetric states since the detector only responds to the intensity signal. Therefore, in the process of multiple image modulations, the stability of the image directly affects the accuracy of the polarimetry, and the high-precision registration of the modulated frames is particularly critical for accurate imaging polarimetry. However, the traditional image registration algorithm cannot serve for high-precision coronal image registration due to low signal-to-noise ratio.

In order to improve the registration accuracy, the researchers proposed the idea of image enhancement based on blind deconvolution combined with noise adaptive fuzzy equalization algorithms. After image enhancement, the cross-correlation registration algorithm achieves better results.

To diagnose coronal physical state, the researchers studied the polarization brightness of the red coronal line data and the MLSO/KCor data to diagnose the structural features and polarimetric information of the inner corona. They found that the polarization brightness of the red coronal line has overall features similar to that of the MLSO/KCor data, and shows more details, such as the cavity, polar plume, and interplume characteristics.

An interesting finding is that the polarization brightness of the red coronal line changes drastically along with the radial height across the cavity above the solar west limb, while neither the K-corona nor the electron density changes obviously. This seems to be a piece of possible evidence indicating that the red coronal line is not unpolarized indeed in the cavity region.

Unfortunately, as there was no corresponding continuum polarization observation around the red coronal line during the 2017 total solar eclipse, the above-proposed incidences cannot be identified. More accurate polarimetry of the coronal red-line is needed.