Recent developments for Balmer line photopolarimetry

Mean longitudinal magnetic fields of stars are determined from observations of circular polarisation in their spectral lines. Two methods have been extensively used: the "photographic" technique, based on consideration of the polarisation of metallic lines, and Balmer line photopolarimetry, which relies on polarisation in the wings of hydrogen Balmer lines. For a number of Ap stars, these two methods yield different values of the longitudinal field. Recent theoretical developments on hydrogen line formation indicate that determinations of the longitudinal field by Balmer line photopolarimetry have so far been based on an incorrect interpretation of the observed polarisation. Using an appropriate description of the formation of the polarised hydrogen Stark broadened profiles, we have revisited this interpretation. Our improved treatment of hydrogen line formation is required to understand the reasons for the differences between existing photographic and photopolarimetric measurements of the magnetic field.