4.5 Article

Probing photospheric magnetic fields with new spectral line pairs

Journal

ASTRONOMY & ASTROPHYSICS
Volume 608, Issue -, Pages -

Publisher

EDP SCIENCES S A
DOI: 10.1051/0004-6361/201731261

Keywords

line: profiles; line: formation; Sun: magnetic fields; Sun: photosphere; polarization

Funding

  1. Alexander von Humboldt Foundation
  2. European Research Council (ERC) under the European Union's Horizon research and innovation programme [695075]
  3. BK21 plus programme through the National Research Foundation (NRF) - Ministry of Education of Korea

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Context. The magnetic line ratio (MLR) method has been extensively used in the measurement of photospheric magnetic field strength. It was devised for the neutral iron line pair at 5247.1 angstrom and 5250.2 angstrom (5250 angstrom pair). Other line pairs as well-suited as this pair have not been reported in the literature. Aims. The aim of the present work is to identify new line pairs useful for the MLR technique and to test their reliability. Methods. We used a three-dimensional magnetohydrodynamic 3D MHD simulation representing the quiet Sun atmosphere to synthesize the Stokes profiles. Then, we applied the MLR technique to the Stokes V profiles to recover the fields in the MHD cube both at original resolution and after degrading with a point spread function. In both these cases, we aim to empirically represent the field strengths returned by the MLR method in terms of the field strengths in the MHD cube. Results. We have identified two new line pairs that are very well adapted to be used for MLR measurements. The first pair is in the visible, Fe I 6820-6842 angstrom, whose intensity profiles have previously been used to measure stellar magnetic fields, and the other pair is in the infrared (IR), Fe I 15 534-15 542 angstrom. The lines in these pairs reproduce the magnetic fields in the MHD cube rather well and, in fact, somewhat better than the original 5250 angstrom pair. Conclusions. The newly identified line pairs complement the old pairs. The lines in the new IR pair, because of their higher Zeeman sensitivity, are ideal for the measurement of weak fields. The new visible pair works best above 300 G. The new IR pair, due to its large Stokes V signal samples more fields in the MHD cube than the old IR pair at 1 : 56 mu m, even in the presence of noise, and hence likely also on the real Sun. Owing to their low formation heights (100-200 km above tau(5000) = 1), both the new line pairs are well suited for probing magnetic fields in the lower photosphere.

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