Overstability of magnetic flux tubes in the Eddington approximation

The behavior of overstable modes of a magnetic flux tube embedded in a stellar atmosphere is studied. The Eddington approximation for the radiative transfer is applied and the effects of radiation losses on the stability pattern are discussed. It is found that the diffusion approximation leads to a considerable overestimation of the growth rates of the unstable modes. The results are applied to an actual flux tube under solar-type condition, and the results and their consistency with the various approximations introduced in the model are discussed.