On the possibility of detecting helium D3 line polarization with Metis

Heinzel, P., Štěpán, J., Bemporad, A., Fineschi, S., Jejčič, S., Labrosse, N. and Susino, R. (2020) On the possibility of detecting helium D3 line polarization with Metis. Astrophysical Journal, 900(1), 8. (doi: 10.3847/1538-4357/aba437)

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Abstract

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 Lα 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 D3 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 D3 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 D3 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 D3 line emission, while depolarization at 10 G amounts roughly to 10%.

Item Type:Articles
Additional Information:The authors acknowledge support from grant Nos. 19-17102S and 19-20632S of the Czech Funding Agency and from RVO:67985815 project of the Astronomical Institute of the Czech Academy of Sciences. The Czech contribution to Metis was funded by ESA-PRODEX. S.J. acknowledges support from the Slovenian Research Agency No. P1-0188. N.L. acknowledges support from the Science and Technology Facilities Council (STFC) Consolidated grant No. ST/ P000533/1.
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Labrosse, Dr Nicolas
Authors: Heinzel, P., Štěpán, J., Bemporad, A., Fineschi, S., Jejčič, S., Labrosse, N., and Susino, R.
College/School:College of Science and Engineering > School of Physics and Astronomy
Journal Name:Astrophysical Journal
Publisher:IOP Publishing
ISSN:0004-637X
ISSN (Online):1538-4357
Published Online:27 August 2020

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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
173869Consolidated Grant in Solar PhysicsLyndsay FletcherScience and Technology Facilities Council (STFC)ST/P000533/1P&S - Physics & Astronomy