Zh. Fan et Yq. Lou, SWING AMPLIFICATION OF FAST AND SLOW DENSITY WAVES IN THIN MAGNETIZEDGASEOUS DISCS, Monthly Notices of the Royal Astronomical Society, 291(1), 1997, pp. 91-109
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.