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

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 \%.

arXiv comment11 pages, 6 figures, accepted for publication in ApJ

References in corpus (9)

• Physics of Solar Prominences: I - Spectral Diagnostics and Non-LTE Modelling
• Advanced Forward Modeling and Inversion of Stokes Profiles Resulting from the Joint Action of the Hanle and Zeeman Effects
• Mapping the magnetic field of flare coronal loops
• Three-dimensional Radiative Transfer Modeling of the Polarization of the Sun's Continuous Spectrum
• Low polarized emission from the core of coronal mass ejections
• Measurements with STEREO/COR1 data of drag forces acting on small-scale blobs falling in the intermediate corona
• Hot prominence detected in the core of a Coronal Mass Ejection: III. Plasma filling factor from UVCS Lyman-$\alpha$ and Lyman-$\beta$ observations
• The eruption of a prominence carrying coronal flux rope: forward synthesis of the magnetic field strength measurement by the COronal Solar Magnetism Observatory Large Coronagraph
• Visibility of Prominences using the HeI D3 Line Filter on PROBA-3/ASPIICS Coronagraph

Cited by in corpus (2)

• A comparative study of two X2.2 and X9.3 solar flares observed with HARPS-N: Reconciling Sun-as-a-star spectroscopy and high-spatial resolution solar observations in the context of the solar-stellar connection
• Polarimetric Studies of a Fast Coronal Mass Ejection