Swing amplification of fast and slow density waves in thin magnetized gaseous discs

Abstract

The excitation and time evolution of magnetohydrodynamic (MHD) density waves are studied in a differentially rotating thin gaseous disc embedded with an azimuthal magnetic field. This analysis shows that both fast and slow MHD density waves are amplified when they swing from leading 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 strong differential rotation, while slow MHD density waves are expected to manifest preferentially in discs of almost rigid rotation. Surface mass density and magnetic field perturbations associated with fast MHD density waves are roughly in phase. A distinct feature of slow MHD density waves is that at a fixed spatial point, there is a significant phase difference δ (i.e. π/2 < δ < π) between the azimuthal magnetic field and surface mass density perturbations during almost the entire evolutionary period. For trailing slow MHD density waves propagating inwards from the corotation, the azimuthal magnetic field perturbation leads spatially the surface mass density perturbation by δ in phase. This feature of slow MHD density waves may explain the approximate anticorrelation between the magnetic field spiral arms seen in polarized radio emissions and the optical spiral arms of the nearby galaxy NGC 6946.