MAGNETIC FLUX CONCENTRATION AND ZONAL FLOWS IN MAGNETOROTATIONAL INSTABILITY TURBULENCE
MAGNETIC FLUX CONCENTRATION AND ZONAL FLOWS IN MAGNETOROTATIONAL INSTABILITY TURBULENCE
复制标题
磁旋转不稳定湍流中的磁通量集中和纬向流动
DOI：
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发表时间：
2014
期刊：
影响因子：
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通讯作者：
J. Stone
中科院分区：
文献类型：
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作者：
X. Bai;J. Stone
吸积盘可能是由外部垂直磁通量，这提高了通过磁旋转不稳定性（MRI）的湍流水平线程。使用剪切盒模拟，我们发现，这样的外部磁通量也强烈增强带状径向密度变化的幅度称为纬向流。此外，我们报告说，垂直磁通量强烈集中向低密度区域的纬向流。平均垂直磁场在低密度区可以增加一倍以上，在某些情况下，在高密度区可以减少到几乎为零。在理想磁流体中，磁通量集中的尺度可以达到几个圆盘尺度高度。在具有强双极扩散的非理想MHD状态下，磁通量集中到轴对称薄壳中，其尺寸通常小于半个标度高度。我们表明，磁通量的集中是密切相关的事实，即MRI湍流的湍流扩散率是各向异性的。除了传统的欧姆样湍流电阻率，我们发现，有一个垂直速度和水平磁场波动，产生一个平均电场的作用，以反扩散的垂直磁通量之间的相关性。各向异性的湍流扩散率具有类似的霍尔效应，并可能有重要的意义，在吸积盘的磁通量输运。磁通量集中的物理起源可能与磁重联之后的通道流的发展有关，磁重联的作用是降低局部区域的质量通量比。增强的纬向流与磁通量集中的关联可能导致原行星盘中的全球压力隆起，这有助于捕获尘埃颗粒并促进行星形成。
Accretion disks are likely threaded by external vertical magnetic flux, which enhances the level of turbulence via the magnetorotational instability (MRI). Using shearing-box simulations, we find that such external magnetic flux also strongly enhances the amplitude of banded radial density variations known as zonal flows. Moreover, we report that vertical magnetic flux is strongly concentrated toward low-density regions of the zonal flow. Mean vertical magnetic field can be more than doubled in low-density regions, and reduced to nearly zero in high-density regions in some cases. In ideal MHD, the scale on which magnetic flux concentrates can reach a few disk scale heights. In the non-ideal MHD regime with strong ambipolar diffusion, magnetic flux is concentrated into thin axisymmetric shells at some enhanced level, whose size is typically less than half a scale height. We show that magnetic flux concentration is closely related to the fact that the turbulent diffusivity of the MRI turbulence is anisotropic. In addition to a conventional Ohmic-like turbulent resistivity, we find that there is a correlation between the vertical velocity and horizontal magnetic field fluctuations that produces a mean electric field that acts to anti-diffuse the vertical magnetic flux. The anisotropic turbulent diffusivity has analogies to the Hall effect, and may have important implications for magnetic flux transport in accretion disks. The physical origin of magnetic flux concentration may be related to the development of channel flows followed by magnetic reconnection, which acts to decrease the mass-to-flux ratio in localized regions. The association of enhanced zonal flows with magnetic flux concentration may lead to global pressure bumps in protoplanetary disks that helps trap dust particles and facilitates planet formation.