Dispersion of long MHD waves in a finely structured plasma

The propagation of long magnetohydrodynamic waves (with wavelength lambda) in a plasma characterized by small scale (of the order of a), large amplitude nonuniformities of its density and of the magnetic field is investigated in conditions where lambda >> a. The introduction of a suitable averaging procedure over an intermediate scale allows one to obtain the relevant dispersion relation for the long waves which manifests new properties with respect to those of a uniform plasma. Moreover, it is shown that, if the plasma nonuniformities are spatially distributed with a broad spectrum, within a finite interval of numerical values, a collisionless mechanism of damping may affect the slow mode. The results of this work are of concern both For astrophysical and laboratory plasmas.