PARAMETRIC MODULATION OF MAGNETOACOUSTIC WAVES IN A MAGNETIC-FIELD TUBE BY AN EXTERNAL WAVE (CORRECTED VERSION OF NU154)

A mechanism of the nonresonant formation of the spectrum for magnetoac oustic waves, coming out of a magnetic field tube, by an external acou stic wave is investigated. The external pressure perturbation, which p ropagates along the tube, deforms the boundary of the tube, leading to a change in its cross-section and Alfven velocity. Such a system is a dynamical resonator for internal waves. The low-frequency part of the spectrum of the waves, which are trapped in the resonator, is determi ned by the spectrum of an external wave, whereas the high-frequency pa rt is determined by conditions of the trapping. An analysis of the spe ctrum of magnetoacoustic waves formed by this mechanism allows one to determine a number of parameters for the magnetic field tube in the so lar atmosphere.