Dynamics of fast magnetosonic perturbations close to an X-X separatrix

The structure of magnetosonic perturbations around a pair of X lines of a t oroidal magnetic configuration is analysed in the framework of ideal MHD, b y means of a WKB technique. The conditions for wave trapping are investigat ed. It is shown that the ware equation for the displacement is separable in the vicinity of the X lines. The relevant eigenvalues and the different ty pes of ray trajectories are discussed.