The profile of a stellar spectral line is formed by the transfer of radiation through the atmosphere by atomic processes in different chemical elements distributed usually unequally over the surface of a magnetic star.

The theory of model atmospheres accounts for all possible physical conditions. Usually one assumes chemical homogeneity with a plane parallel atmosphere. The resulting line profile, however, is strongly deformed by the geometrical influence of the topographic element distribution and the magnetic surface field structure as well as the projection onto the line of sight of the outgoing radiation from all surface points and its integration over the visible disk.

Line formation by the geometry of projection and element distribution is used for the inverse procedure of Doppler Imaging by V.L. Khokhlova and her followers. We consider here only the influence of the magnetic field on the line profile including the Stokes parameters $I, Q, U, V$, which we study separately from other effects. Thus as a function of the stellar aspect due to rotation the magnetic field and the projection make a symmetric “plane atmosphere profile” asymmetric.

Line profile deformation by the magnetic field leads to a fatal consequence for the traditional measurement of stellar magnetic fields by the Zeeman displacement of the circularly polarized $\sigma$-components. The large scatter of measuring points is partly due to the asymmetry of the line profiles!To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html