Diffusion and magnetic field effects on stellar surfaces

Chemically peculiar stars are ideal astrophysical laboratories for furnishing a wide range of theoretical and observational aspects of our knowledge of the physical processes in stars. Recent dramatic improvements in the quality of the observational data and refinements of the modelling techniques led to an emergence of a new branch of stellar astrophysics which is focused on the reconstruction and the understanding of the origin of the three-dimensional structures in stellar surface layers. In this contribution I present an overview of recent results of the detailed modelling of the chemical nonuniformities, the magnetic and the pulsation velocity fields in the atmospheres of A stars. New Doppler imaging analyses of the magnetic field and the chemical inhomogeneities reveal an unexpected complexity of the surface formations and suggest that nonmagnetic phenomena play an important role in shaping the geometry of chemical spots. Consideration of the line profile shapes observed at high spectral and time resolution has made it possible to probe the radial dependence of the chemical abundances and the pulsation characteristics of cool pulsating Ap stars. An extension of Doppler mapping to the reconstruction of non-radial stellar oscillation structure delivers a solution of the long-standing problem of the pulsational geometry of roAp stars and helps to elucidate the interrelation between the pulsations, the magnetic field and the stellar rotation.