The observation and interpretation of the polarized radiation of spectral lines formed in the solar atmosphere have become, over the past
century, the diagnostic method of choice for the investigation of solar magnetism. Long before the advent of quantum mechanics, the effect of a
magnetic field on a radiating atom had been invoked by G.E. Hale to explain the splitting and polarization of spectral lines observed in sunspots. Ever since, the steadfast improvement of our observational tools and of our
understanding of the atomic physics underlying the production of polarized radiation have opened new exciting pathways to the diagnostics of
electro-magnetic fields on the sun.
In this lecture, I will review the basic physical concepts necessary to understand how polarized radiation is produced on the sun, with a particular emphasis on the process of resonance scattering, where the quantum-mechanical behavior of a radiating atom subject to an external field can be unmistakeably recognized in the peculiarity of its spectro-polarimetric signature in solar lines. Typical applications to the investigation of solar structures will be illustrated.
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