Depolarizing lines in the Sun's spectrum

The majority of all lines in the solar spectrum depolarize the continuum. Here we present a theoretical analysis of the depolarizing lines. In a parameter survey where we vary the details of line formation such as height of formation, we find that absorption and scattering lines have entirely different behavior. While the depolarization by absorption lines rapidly decreases with increasing height of formation, the depolarization by scattering lines increases with height of formation. We interpret the results in terms of a simple qualitative model that contains the relevant physics and support it with quantitative calculations. The main relevant quantities are the anisotropy of the radiation field, which varies over the line profile, and the probability that a photon undergoes polarized scattering above the height where the atmosphere becomes optically thin. For lines with intrinsic polarization we find that the coupling between continuum and line polarization can be neglected.