Magnetic perturbations to the acoustic modes of roAp stars

We study the effect of a large-scale surface magnetic field on the non-radi al acoustic modes of roAp stars. Special attention is given to the use of a variational principle which is used for determining the shifts in the freq uencies with relative ease, enabling us to avoid having to calculate the pe rturbed eigenfunctions. With knowledge of the frequency shifts we then esti mate the eigenfunctions in a simpler, albeit approximate way. The results i ndicate frequency shifts of the order of few mu HZ, which depend on the ord er, degree and azimuthal order of the mode. The loss of energy through Alfv en waves is also estimated from the imaginary parts of the frequency shifts . The results indicate that the loss is particularly high near specific fre quencies. This might indicate the presence of a selection effect, which cou ld make some modes more likely to be excited than others. However, our resu lts do not explain why the modes observed appear always to be aligned with the axis of the magnetic field. Finally, the estimated perturbed eigenfunct ions contain strong components of spherical harmonics that differ from thos e of the original unperturbed modes.