... I D₃

Appendix A is only available in electronic form at http://www.edpsciences.org

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... Research

The National Center for Atmospheric Research is sponsored by the National Science Foundation.

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...U

We use the customary definition of positive Q indicating linear polarization tangent to the solar limb.

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... frame

For a graphical definition of the local and LOS reference frames, as well as of the angles in each one of those frames, please refer to Fig. 1 of Casini (2002) or López Ariste & Casini (2002).

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... regime

This is strictly true only for multi-level atoms where quantum interferences between levels with different total angular momentum are negligible (see, e.g., Trujillo Bueno et al. 2002).

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