Center-to-limb variation of scattering polarization in molecular solar lines: Observations and modeling

We present center-to-limb measurements of the scattering polarization observed in molecular lines of C(2) and MgH in the spectral range between 515.7 and 516.1 nm, together with a radiative transfer model for the formation of these lines. The observations were performed in July 2000 with THEMIS inside the south polar limb. We were able to measure the polarization at distances between 1 arcsec and 50 arcsec from the solar limb. The lines appear as very weak absorption features in the intensity spectrum but their linear polarization clearly dominates in the polarization spectrum. We introduce here a simple radiative transfer model which allows to interpret the observed center-to-limb variations of both the intensity and linear polarization. The basic assumption is that molecular lines are formed higher in the photosphere than the continuous photospheric radiation. Molecules are thus illuminated by the polarized continuum photospheric radiation field. We account for a possible Hanle effect due to weak unresolved magnetic fields but we neglect depolarizing collisions. The model depends on four parameters which are determined by fitting both the intensity and polarization in 9 molecular lines of the observed spectral domain. Making use of the differential Hanle effect in the different lines of C(2) we show that the C2 lines are affected by the Hanle effect due to a weak unresolved magnetic field. Its mean strength is on the order of 15 Gauss in the upper photosphere and increases to values on the order of 50 Gauss at larger depths. These results are in good agreement with those derived previously (Faurobert et al. 2001) from the linear polarization of the SrI 460.7 nm line which was observed simultaneously. Such a weak field has almost no effect on the MgH lines.