Solar EUV/FUV line polarimetry: observational parameters and theoretical considerations

Abstract

Several mechanisms can induce a detectable amount of linear polarization (&square; 1%) in spectral lines emitted by the outer solar atmosphere at EUV/FUV wavelengths (100 A< A < 1500 A): (1) Polarization in FUV lines (up to 20%) can be originated by resonance scattering of radiation anisotropically illuminating the emitting atoms. Modifications of this polarization can then result from the presence of a magnetic field (Hanle effect); (2) Impact line polarization can arise from anisotropic collisional excitation of the EUV emitting atoms by particles (electrons, protons) with non-Maxwellian velocity distributions. We suggest how new technological developments associated with the production of ultra-smooth, low scatter flow-polished mirror substrates and high quality multilayer and interference film coatings can make possible some exciting new optical instruments which should permit observations of these polarization effects. Polarization measurements have not previously been obtained in these wavelength regimes. We give the observational parameters for the development of all refledive FUV coronagraph/polarimeter and EUV imaging polarimeter instruments. A coronagraph/polarimeter, operating at hydrogen Lyman a, could provide - via the Hanle effect - the first direct measurements of coronal vector magnetic fields, of relevance to numerous fundamental questions in modern solar physics. A multilayer EUV imaging polarimeter, operating at EUV helium lines (e.g., 304 A, 584 A), could provide information on impact polarization phenomena, relevant to questions concerning the relative importance of thermal and non-thermal processes in solar flares. The reflecting polarization analyzers for these instruments will operate at the Brewster angle and will be coated with advanced thin film interference coatings or EUV multilayer coatings. We describe some new types of EUV/FUV polarimeters based on these polarization optics.