Coronal Photopolarimetry with the LASCO-C2 Coronagraph over 24 Years [1996-2019] -- Application to the K/F Separation and to the Determination of the Electron Density

TL;DR

This paper thoroughly characterizes the LASCO-C2 coronagraph's polarimetric channel over 24 years, enabling improved separation of K and F corona components and more accurate electron density measurements.

Contribution

It provides a detailed calibration and correction methodology for the LASCO-C2 polarimetric data, enhancing the accuracy of coronal polarization and electron density analysis.

Findings

Polarization of the white-light corona characterized over two solar cycles.

Effective separation of K and F corona components achieved.

Electron density measurements validated against eclipse data.

Abstract

We present an in-depth characterization of the polarimetric channel of the Large-Angle Spectrometric COronagraph LASCO-C2 onboard the Solar and Heliospheric Observatory (SoHO). The polarimetric analysis of the white-light images makes use of polarized sequences composed of three images obtained though three polarizers oriented at +60{\deg}, 0{\deg} and -60{\deg}, complemented by a neighboring unpolarized image, and relies on the formalism of Mueller. The Mueller matrix characterizing the C2 instrument was obtained through extensive ground-based calibrations of the optical components and global laboratory tests. Additional critical corrections were derived from in-flight tests relying prominently on roll sequences and on consistency criteria, mainly the tangential direction of polarization. Our final results encompass the characterization of the polarization of the white-light corona, of its polarized radiance, of the two-dimensional electron density, and of the K-corona over two solar cycles. They are in excellent agreement with measurements obtained at several solar eclipses except for slight discrepancies affecting the innermost part of the C2 field-of-view, probably resulting from an imperfect removal of the bright diffraction fringe surrounding the occulter.