The physical foundations of stellar magnetic field diagnosis from polarimetric observations of hydrogen lines

We have completed a new study of the formation of hydrogen lines in dense m agnetized plasmas. We have developed a global formalism including the effec ts of the magnetic field and of the motional electric field. The latter, re sulting from the motion of the radiating hydrogen atom in the magnetic fiel d, modifies the structure of the matrices appearing in the equation of tran sfer of the Stokes vector and introduces a coupling between microscopic ("i ntrinsic") and macroscopic (Doppler) contributions to the line profiles. We have used this new treat ment of line formation to revisit the interpretat ion of photopolarimetric (Stokes I and V) observations of Balmer lines in t erms of mean longitudinal magnetic field.