On the Possibility of Detecting Helium D3 Line Polarization with Metis

Metis, the space coronagraph on board the Solar Orbiter, offers us new capabilities for studying eruptive prominences and coronal mass ejections (CMEs). Its two spectral channels, hydrogen L alpha and visible light (VL), will provide for the first time coaligned and cotemporal 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 diagnostic 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 and 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 measuringQ/IandU/Ipolarization degrees and we show their dependence on temperature and magnetic field. AtT = 30,000 K we find a significant lowering ofQ/Iwhich is due to strongly enhanced D(3)line emission, while depolarization at 10 G amounts roughly to 10%.