Study of the polarization produced by the Zeeman effect in the solar Mg i b lines.

The next generation of solar observatories aim to understand the magnetism of the solar chromosphere. Therefore, it is crucial to understand the polarimetric signatures of chromospheric spectral lines. For this purpose, we here examine the suitability of the three Fraunhofer Mg i b 1, b 2, and b 4lines at 5183.6, 5172.7, and 5167.3 Å, respectively. We start by describing a simplified atomic model of only six levels and three line transitions for computing the atomic populations of the 3p-4s (multiplet number 2) levels involved in the Mg i b line transitions assuming non-local thermodynamic conditions and considering only the Zeeman effect using the field-free approximation. We test this simplified atom against more complex ones finding that, although there are differences in the computed profiles, they are small compared with the advantages provided by the simple atom in terms of speed and robustness. After comparing the three Mg i lines, we conclude that the most capable one is the b 2line as b 1forms at similar heights and always shows weaker polarization signals, while b 4is severely blended with photospheric lines. We also compare Mg i b 2with the K i D 1and Ca ii 8542 Å lines finding that the former is sensitive to the atmospheric parameters at heights that are in between those covered by the latter two lines. This makes Mg i b 2an excellent candidate for future multiline observations that aim to seamlessly infer the thermal and magnetic properties of different features in the lower solar atmosphere. [ABSTRACT FROM AUTHOR]