Measurements of the degree of polarization of light in the spectral lines emitted by gas near the Sun's limb (its outer edge) can be used to investigate the electron densities and magnetic field strengths in the solar atmosphere; these quantities are important for determining the balance and transport of energy through the Sun's atmosphere. Recent measurements revealed a surprising degree of polarization in the sodium doublet; these observations have remained an enigma. Here 1 report a mechanism that may explain these observations, in which it is assumed that the populations of the magnetic sublevels of the electronic ground state of the sodium atom are not equal: this leads to ground-level atomic polarization. This mechanism explains very well the observed line shapes, and implies that depolarization does not occur in the solar chromosphere, which would seem to rule out the existence of turbulent magnetic fields and of horizontal, canopy-like fields stronger than ～0.01 G. This is difficult to understand, because there is substantial evidence from other types of observation for both types of field. There are obviously aspects of the Sun's atmosphere that remain very poorly understood.
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