Diagnostic capabilities of spectropolarimetric observations for understanding solar phenomena

I. Zeeman-sensitive photospheric lines

¹ Rosseland Centre for Solar Physics, University of Oslo, PO Box 1029, Blindern 0315 Oslo, Norway
² Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern 0315 Oslo, Norway
³ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
e-mail: carlos.quintero@iac.es
⁴ Departamento de Astrofísica, Univ. de La Laguna, La Laguna, Tenerife 38205, Spain
⁵ Theoretical Astrophysics, Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden
⁶ Instituto de Astrofísica de Andalucía (CSIC), Apartado de Correos 3004, 18080 Granada, Spain
⁷ Univ. Coimbra, IA, DF, OGAUC, Coimbra, Portugal
⁸ National Solar Observatory, University of Colorado Boulder, 3665 Discovery Drive, Boulder, CO 80303, USA
⁹ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

Received: 14 February 2020
Accepted: 18 May 2021

Abstract

Future ground-based telescopes will expand our capabilities for simultaneous multi-line polarimetric observations in a wide range of wavelengths, from the near-ultraviolet to the near-infrared. This creates a strong demand to compare candidate spectral lines to establish a guideline of the lines that are most appropriate for each observation target. We focused in this first work on Zeeman-sensitive photospheric lines in the visible and infrared. We first examined their polarisation signals and response functions using a 1D semi-empirical atmosphere. Then we studied the spatial distribution of the line core intensity and linear and circular polarisation signals using a realistic 3D numerical simulation. We ran inversions of synthetic profiles, and we compared the heights at which we obtain a high correlation between the input and the inferred atmosphere. We also used this opportunity to revisit the atomic information we have on these lines and computed the broadening cross-sections due to collisions with neutral hydrogen atoms for all the studied spectral lines. The results reveal that four spectral lines stand out from the rest for quiet-Sun and network conditions: Fe I 5250.2, 6302, 8468, and 15 648 Å. The first three form higher in the atmosphere, and the last line is mainly sensitive to the atmospheric parameters at the bottom of the photosphere. However, as they reach different heights, we strongly recommend using at least one of the first three candidates together with the Fe I 15 648 Å line to optimise our capabilities for inferring the thermal and magnetic properties of the lower atmosphere.