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Optical Engineering

Imaging polarimeters for solar EUV astronomy
Author(s): Richard B. Hoover; Silvano Fineschi; Juan M. Fontenla; Arthur B. C. Walker
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Paper Abstract

αWe describe new EUV/FUV (100 Å ≤ λ ≤ 1500 Å) polarimeter instrument concepts for solar research. These instruments are designed to observe linear polarization in EUV/FUV spectral lines originating in the outer solar atmosphere, specifically: (1) a new coronagraph/polarimeter operating at 1215.7 Å (neutral hydrogen Lyman ), which could observe this line in the near solar corona and lead to the first direct measurements of both strength and direction of coronal magnetic fields and (2) a new multilayer EUV imaging polarimeter, operating at wavelengths of strong helium emission lines (e.g., 304 Å, 584 Å), which could observe impact polarization phenomena and provide information concerning the relative importance ofthermal and nonthermal processes in solarflares. The emission mechanisms we will address with these instruments include resonance scattering and impact polarization. Resonance scattering of chromospheric radiation anisotropically illuminating the emitting atoms in the corona can produce up to 20% linear polarization in FUV coronal lines. Modifications, via the Hanle effect, of this polarization would result from the presence of a magnetic field. In the EUV, detectable polarization may be produced by impact polarization, which results from anisotropic collisional excitation of the emitting atoms by particles (electrons, protons) with non-Maxwellian velocity distributions produced during flares. These coronagraph/polarimeter instruments employ all-reflective optical systems utilizing ultrasmooth, low-scatter normal incidence mirrors and reflective polarization analyzers comprised of advanced flow-polished substrates with state-of-the-art thin film FUV interference and EUV multilayer coatings. The reflecting polarization analyzers operate at the Brewster angle. We discuss several instrument configurations and provide theoretical calculations and performance predictions for coronagraph/polarimeter instruments utilizing an optical design similar to the Ritchey-Chrétien EUV/FUV telescopes developed for flight on the Stanford/MSFC/LLNL Multi-Spectral Solar Telescope Array (MSSTA).

Paper Details

Date Published: 1 August 1991
PDF: 8 pages
Opt. Eng. 30(8) doi: 10.1117/12.55923
Published in: Optical Engineering Volume 30, Issue 8
Show Author Affiliations
Richard B. Hoover
Silvano Fineschi, Harvard-Smithsonian Ctr. for Astrophysics (Italy)
Juan M. Fontenla, Univ. of Alabama in Huntsville (United States)
Arthur B. C. Walker, Stanford Univ. (United States)

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