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arXiv 0906.0345astro-ph.HEastro-ph.GA
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大尺度B场在吸积盘中的平流/扩散

Advection/Diffusion of Large-Scale B-Field in Accretion Disks

R. V. E. Lovelace, D. M. Rothstein, G. S. Bisnovatyi-Kogan

摘要:

星系核和恒星质量系统的活动涉及到黑洞的吸积盘,被认为是由于(1)小规模的湍流磁场在磁盘(由于磁旋转不稳定性或MRI),这给出了一个大的粘性增强吸积,和(2)大规模的磁场,这引起物质流出和/或电磁射流从磁盘,这也增强吸积。这幅图的一个重要问题是,增强的粘度伴随着增强的磁扩散率,其作用是防止建立一个显着的大规模场。最近的工作指出,磁盘的表面层是非湍流的,因此具有高传导性(或非扩散性),因为磁共振成像在磁盘中被抑制得很高,其中磁压力和辐射压力大于热压。在这里,我们计算的垂直($z$)配置文件的固定吸积流(与径向和方位角分量),和配置文件的大规模,磁场考虑到湍流的粘度和扩散率由于MRI和湍流消失在磁盘的表面的事实。我们发现的固定的解决方案表明,一个弱($β> 1$)的大规模的字段不扩散的建议由早期的工作。

英文摘要:

Activity of the nuclei of galaxies and stellar mass systems involving disk accretion to black holes is thought to be due to (1) a small-scale turbulent magnetic field in the disk (due to the magneto-rotational instability or MRI) which gives a large viscosity enhancing accretion, and (2) a large-scale magnetic field which gives rise to matter outflows and/or electromagnetic jets from the disk which also enhances accretion. An important problem with this picture is that the enhanced viscosity is accompanied by an enhanced magnetic diffusivity which acts to prevent the build up of a significant large-scale field. Recent work has pointed out that the disk's surface layers are non-turbulent and thus highly conducting (or non-diffusive) because the MRI is suppressed high in the disk where the magnetic and radiation pressures are larger than the thermal pressure. Here, we calculate the vertical ($z$) profiles of the stationary accretion flows (with radial and azimuthal components), and the profiles of the large-scale, magnetic field taking into account the turbulent viscosity and diffusivity due to the MRI and the fact that the turbulence vanishes at the surface of the disk. The stationary solutions we find indicate that a weak ($β> 1$) large-scale field does not diffuse away as suggested by earlier work.

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