Detection of polarization neutral points in observations of the combined corona and sky during the 21 August 2017 total solar eclipse

Abstract

We report the results of polarimetric observations of the total solar eclipse of Aug. 21 2017 from Rexburg, ID (USA). We use three synchronized DSLR cameras with polarization filters oriented at 0, 60 and 120 degrees to provide high-dynamic-range RGB polarization images of the corona and surrounding sky. We measure tangential coronal polarization and vertical sky polarization, both as expected. These observations provide detailed detections of polarization neutral points above and below the eclipsed Sun where the coronal polarization is cancelled by the sky polarization. We name these special polarization neutral points after Minnaert and Van de Hulst.

Coronal Electron Densities derived with Images acquired during the 21 August 2017 Total Solar Eclipse

“…coronal regions, and in good agreement with values provided for instance by Snik et al (2020) (see their Fig. 2).…”

“…In this work, the mosaic images acquired with different polarizations have been combined, to measure the degree of linear polarization p [%], and the three components [I,Q,U] of the Stokes vector. Despite the presence of a few artifacts (ghosts due to reflections from the polarizing filter, and a cross-like divergent pattern where solutions for the Qcomponent of Stokes vector diverge), the resulting values of p are in nice agreement with those provided for instance by Snik et al (2020) for the same TSE. Relative radiometric calibration has been performed re-scaling measurements of polarized brightness pB obtained here from from TSE with those provided by the Mauna Loa K-Cor coronagraph (MLSO), showing a very good agreement both in the latitudinal distribution and at different altitudes up to 2 R sun , with very good signal-to-noise ratio.…”

“…10. The two images show, unfortunately, the presence of a ghost (due to unavoidable internal reflections from the linear polarizing filter); very similar features were present for instance also in images acquired by Snik et al (2020) (see their Fig. 2).…”

“…Although these works are mostly addressed to amateur astronomers, teachers and educators, they also showed that, even though not explicitly designed for scientific applications, DSLR cameras can nevertheless produce high-quality data only a minimal investment of funds. Surprisingly, the above list of published papers shows a very limited number of works doing research with DSLR cameras in solar physics, with the exception of a few recent works focusing on the measurement of plasma physical parameters in a quiescent prominence (Jejčič et al 2014) based on the method by Jejčič & Heinzel (2009), the determination of contact times of solar eclipses and planetary transits across the solar disk (Di Giovanni 2016), the measurement of the apparent variations of the size of the Sun (Trillenberg 2019), and the recent observations of the total solar eclipse (TSE) of 21st August 2017 (Pasachoff et al 2018;Snik et al 2020). In particular, the latter spectacular event was observed by thousands of people as the path of totality crossed the whole US country from coast to coasts, and allowed for the first time to involve the general public on vast citizen science projects, for instance to observe cloud and temperature properties associated with the transit of the eclipse (Dodson et al 2019), to measure the ionospheric response to the variable solar illumination (Frissell et al 2018), to capture (with the "Citizen CATE Experiment"; Penn et al 2020) a time sequence of white-light coronal observations with identical instruments over ∼ 90 minutes of totality, or to collect (with the "Eclipse Megamovie Project"; Hudson et al 2011) all DSLR pictures of the solar eclipse acquired by people across the US to create a movie showing the high-resolution coronal dynamics close to the limb (see Hudson et al 2018;Peticolas et al 2019, for first results).…”

“…Reinisch et al 2018). On the other hand, only a couple of works discussed the scientific research that can be conducted simply with DSLR cameras, to constrain the locations of fainter coronal structures (Pasachoff et al 2018), and to measure the degree of linear polarization (Snik et al 2020). In this work I demonstrate how images acquired during TSE with a single basic DSLR camera equipped with cost effective ND filter and linear polarizer can be analyzed to derive not only beautiful high-resolution images of the corona, but also to calibrate the polarized emission and measure the coronal electron densities.…”