Compositional waves and variations in the atmospheric abundances of magnetic stars

The stars of the middle main sequence often have relatively quiescent outer layers and spot-like chemical structures may develop in their atmospheres. Recent observations show that abundance peculiarities can change as stars evolve on the main sequence and the timescale of these changes lies in a wide range from million years to months. These observations imply that, perhaps, our understanding of diffusion processes at work in magnetic stars is incomplete and a more detailed analysis of these processes is required. In the present paper, we consider diffusion caused by a combined influence of the electric current and the Hall effect. Such diffusion has a number of very particular properties and, generally, can change the surface chemistry of stars in combination with other diffusion processes. For instant, current-driven diffusion is accompanied by a propagation of the special type of waves in which only the impurity number density oscillates. Propagation of such waves changes the shape and size of spots as well as chemical abundances within them. The period of compositional waves depends on the parameters of plasma (magnetic field, electric current, temperature, etc.) and can be different for the waves of different elements. Compositional waves exist in the regions where the magnetic pressure is greater than the gas pressure. These waves can be the reason of variations with different timescales in the abundance peculiarities of magnetic stars.