Abstract
Twenty-five years ago, enigmatic linear polarization signals were discovered in the core of the sodium D_{1} line. The only explanation that could be found implied that the solar chromosphere is practically unmagnetized, in contradiction with other evidences. This opened a paradox that has challenged physicists for many years. Here we present its solution, demonstrating that these polarization signals can be properly explained in the presence of magnetic fields in the gauss range. This result opens a novel diagnostic window for exploring the elusive magnetism of the solar chromosphere.
Highlights
Twenty-five years ago, enigmatic linear polarization signals were discovered in the core of the sodium D1 line
This so-called scattering polarization is due to the presence of atomic level polarization, produced when the atom is illuminated by anisotropic radiation [4,5]
Our results show that linear polarization is produced in the D1 line in the absence of any ground-state polarization, even in the presence of inclined magnetic fields in the gauss range, and that the calculated spectral line polarization is similar to that found in recent high-precision spectropolarimetric observations [12]
Summary
Twenty-five years ago, enigmatic linear polarization signals were discovered in the core of the sodium D1 line. The requirement that the lower solar chromosphere must be practically unmagnetized conflicts with the results from observations in other spectral lines as well as with plasma physics arguments, which instead indicate the presence of magnetic fields in the gauss range in this key interface region of the solar atmosphere [15].
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