SWING AMPLIFICATION OF FAST AND SLOW DENSITY WAVES IN THIN MAGNETIZEDGASEOUS DISCS

The excitation and time evolution of magnetohydrodynamic (MHD) density waves are studied in a differentially rotating thin gaseous disc embe dded with an azimuthal magnetic field. This analysis shows that both f ast and slow MHD density waves are amplified when they swing from lead ing to trailing configurations, but the amplification factors of fast and slow MHD density waves depend differently on the disc differential rotation. Fast MHD density waves tend to be excited in discs of stron g differential rotation, while slow MHD density waves are expected to manifest preferentially in discs of almost rigid rotation. Surface mas s density and magnetic field perturbations associated with fast MHD de nsity waves are roughly in phase. A distinct feature of slow MHD densi ty waves is that at a fixed spatial point, there is a significant phas e difference delta (i.e. pi/2 < delta < pi) between the azimuthal magn etic field and surface mass density perturbations during almost the en tire evolutionary period. For trailing slow MHD density waves propagat ing inwards from the corotation, the azimuthal magnetic field perturba tion leads spatially the surface mass density perturbation by delta in phase. This feature of slow MHD density waves may explain the approxi mate anticorrelation between the magnetic field spiral arms seen in po larized radio emissions and the optical spiral arms of the nearby gala xy NGC 6946.