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

¹ INAF – Astrophysical Observatory of Turin, Via Osservatorio 20, 10025 Pino Torinese, Turin, Italy
e-mail: alessandro.liberatore@inaf.it
² The Catholic University of America at NASA-GSFC, 620 Michigan Ave NE, Washington, DC 20064, USA
³ INAF – Astronomical Observatory of Capodimonte, Salita Moiariello 16, 80131 Naples, Italy
⁴ CNR – Institute for Photonics and Nanotechnologies, Via Trasea 7, 35131 Padua, Italy
⁵ University of Urbino Carlo Bo and INFN, Via Aurelio Saffi 2, 61029 Urbino, Italy
⁶ INAF – Trieste Astronomical Observatory, Via Giambattista Tiepolo 11, 34149 Trieste, Italy
⁷ US Naval Research Laboratory, 4555 Overlook Ave SW, Washington, DC 20375, USA
⁸ University of Padua, Via VIII Febbraio 2, 35122 Padua, Italy
⁹ University of Florence, Via Nello Carrara 1, 50019 Sesto Fiorentino, Florence, Italy
¹⁰ INAF – Astrophysical Observatory of Catania, Via S. Sofia 78, 95123 Catania, Italy
¹¹ ASI – Italian Space Agency, Via del Politecnico, 00133 Rome, Italy
¹² Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
¹³ INAF – Institute for Space Astrophysics and Cosmic Physics, Via Giovanni Celoria 16, 20133 Milan, Italy

Received: 20 May 2022
Accepted: 5 February 2023

Abstract

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 different wavelengths: visible light (VL), within a band ranging from 580 nm to 640 nm, and ultraviolet light, in the HI Lyα 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.