Research Division Seminar
Solving the paradox of the solar sodium D1 polarization

Abstract

25 years ago, a seminal letter was published in Nature where it was concluded that the enigmatic linear polarization signal observed in the solar sodium D1 line implies that the quiet solar chromosphere is practically unmagnetized, in contradiction with other observational inferences and plasma physics arguments. This became known as the paradox of the solar sodium D1 polarization, which has puzzled theoretical physicists for many years, even leading some scientists to question the established quantum theory of radiation-matter interaction. In this talk I will briefly discuss the theoretical basis for this intriguing paradox and present its resolution in terms of a radiative transfer investigation published in Physical Review Letters. The observed linear polarization pattern across the sodium D2 and D1 lines can be reproduced to a remarkable degree in the presence of magnetic fields in the gauss range, if one accounts for the variations in the anisotropy of the solar radiation field over the small spectral interval spanned by the various hyperfine structure components of the sodium D2 and D1 lines. In addition, I will present the results of a series of radiative transfer investigations focused on the D lines of other alkali species, namely K I and Ba II, discussing their interest for probing the magnetism of the solar chromosphere.