Molecular line scattering and magnetic field effects: Resolution of an enigma

The linearly polarized solar spectrum that is produced by coherent scattering processes (also called “the second solar spectrum”) is full of polarizing features due to molecular transitions, in particular from MgH and C2. Their greatly different behavior in comparison with the observed polarization from atomic transitions has presented us with a new enigma: While the scattering polarization in atomic lines is very sensitive to magnetic fields via the Hanle effect and therefore exhibits polarization signatures that vary both spatially and with the solar cycle, the molecular polarization appears to be immune to the influence of magnetic fields. To clarify these issues we here develop a theoretical foundation for polarized molecular scattering, which can serve as a general tool for interpretations of the structures in the second solar spectrum. Intrinsic polarizabilities, line strengths, and effective Landé factors for the different transitions of the P, Q, and R branches of MgH and C2 are calculated. While the intrinsic polarizabilities remain significant, the effective Landé factors are close to zero for the majority of the lines, in contrast to the behavior of atomic lines. This resolves the enigma and indicates how the molecular lines may serve as immutable reference lines against which the atomic lines can be gauged when trying to determine long-term, solar-cycle variations of the magnetic fields via the Hanle effect.