In-flight validation of the Metis visible-light polarimeter coronagraph on board Solar Orbiter

Context. The Metis coronagraph is one of the remote sensing instruments of the ESA-NASA Solar Orbiter mission. The goal for
the instrument is to enable the study of the solar atmosphere and solar wind by simultaneously acquiring images of the solar corona
at two di erent wavelengths: visible light (VL), within a band ranging from 580 nm to 640 nm, and ultraviolet light, in the HI Ly
121:6 10 nm. The visible-light channel of the coronagraph includes a polarimeter with electro-optically modulating liquid crystal
variable retarders to measure the linearly polarized brightness of the K-corona and derive the electron density.
Aims. In this paper, we present the first in-flight validation results of the Metis polarimetric channel together with a comparison to the
on-ground calibrations. This paper seeks to validate the first use of an electro-optical device, the liquid crystal-based polarimeter, in
deep space and within a hard radiation environment.
Methods. We used the orientation of the K-corona’s linear polarization vector during the roll maneuvers of the Space Orbiter spacecraft
for the in-flight calibration.
Results. The Metis coronagraph on board the Solar Orbiter shows good agreement with the on-ground measurements. The in-flight
validation confirms the expected performance of the visible-light channel polarimeter. Furthermore, a comparison between the first
polarized brightness value obtained by Metis and the polarized brightness values obtained by the space-based coronagraph LASCO
and the ground-based coronagraph K-Cor shows the consistency of the Metis calibrated results.