On the Possibilities of Detecting Helium D$_3$ Line Polarization with Metis

TL;DR

This paper explores the potential of the Metis coronagraph on Solar Orbiter to detect and analyze the linear polarization of the helium D3 line in eruptive prominences and CMEs, providing insights into their thermal and magnetic structures.

Contribution

The study predicts the diagnostic capabilities of the helium D3 line polarization detection with Metis, including its sensitivity to temperature and magnetic fields in CMEs.

Findings

Metis VL channel can detect helium D3 emission at 30-50,000 K.

Polarization measurements can reveal magnetic field effects via Hanle effect.

Depolarization of about 10% at 10 G magnetic field strength.

Abstract

Space coronagraph Metis on board of the Solar Orbiter offers us new capabilities for studying eruptive prominences and coronal mass ejections (CME). Its two spectral channels, hydrogen L$\alpha$ and visible-light (VL) will provide, for the first time, co-aligned and co-temporal images to study dynamics and plasma properties of CMEs. Moreover, with the VL channel (580 - 640 nm) we find an exciting possibility to detect the helium D$_3$ line (587.73 nm) and its linear polarization. The aim of this study is to predict the diagnostics potential of this line regarding the CME thermal and magnetic structure. For a grid of models we first compute the intensity of the D$_3$ line together with VL continuum intensity due to Thomson scattering on core electrons. We show that the Metis VL channel will detect a mixture of both, with predominance of the helium emission at intermediate temperatures between 30 - 50,000 K. Then we use the code HAZEL to compute the degree of linear polarization detectable in the VL channel. This is a mixture of D$_3$ scattering polarization and continuum polarization. The former one is lowered in the presence of a magnetic field and the polarization axis is rotated (Hanle effect). Metis has the capability of measuring $Q/I$ and $U/I$ polarization degrees and we show their dependence on temperature and magnetic field. At $T$=30,000 K we find a significant lowering of $Q/I$ which is due to strongly enhanced D$_3$ line emission, while depolarization at 10 G amounts roughly to 10 \%.