Swing amplification of fast and slow density waves in thin magnetized gaseous discs
Swing amplification of fast and slow density waves in thin magnetized gaseous discs
复制标题
薄磁化气盘中快慢密度波的摆动放大
DOI：
10.1093/mnras/291.1.91
复制
发表时间：
1997
影响因子：
4.8
通讯作者：
Y. Lou
中科院分区：
文献类型：
--
作者：
Z. Fan;Y. Lou
研究了嵌入有方位磁场的差旋转薄气盘中磁流体密度波的激发和时间演化。分析结果表明，快速和慢速MHD密度波在由前向后摆动时都会被放大，但快速和慢速MHD密度波的放大因子取决于圆盘的差速旋转。快MHD密度波倾向于在强微分旋转盘中被激发，而慢MHD密度波预计优先出现在几乎刚性旋转的盘中。与快速MHD密度波相关的表面质量密度和磁场扰动大致相同。MHD慢密度波的一个显著特征是，在一个固定的空间点上，几乎在整个演化过程中，方位角磁场和表面质量密度扰动之间存在显著的相位差δ（即π/2 < δ < π）。对于从地穴向内传播的慢磁密度波，方位磁场扰动在空间上领先于表面质量密度的δ相位扰动。慢MHD密度波的这一特征可以解释在偏振射电辐射中看到的磁场旋臂与附近星系NGC 6946的光学旋臂之间的近似反相关关系。
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.