In the magnetosphere, the two basic hydromagnetic modes, the isotropic or poloidal and the guided or toroidal, must always be coupled. This problem of coupled propagation must be solved before long period magnetic variations can be explained or utilized. The problem has added interest since the two modes are so dissimilar and the toroidal mode is peculiar by itself. Mode coupling has been studied for the very useful model of the MHD wedge. This geometry has the virtue that it avoids cumbersome

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... athematical formalism and yet retains the essential physical difficulty inherent in the magnetospheric problem to which the results may be applied. Among the steady state solutions, to be presented, are a distinctive type in which the eigenmodes can exist only in prescribed regions of the system. The eigenperiods themselves aid in determining the limits of the system to which they apply. This unusual behavior is associated with oscillating sheet currents on field line shells of a particular characteristic period, which reflect waves of the same period. Such a phenomenon may give rise to ducting of definite bands of frequencies of hydromagnetic waves. It also poses several important problems in excitation, growth and dissipation of the wave energy.