TRANSPORT OF LARGE-SCALE POLOIDAL FLUX IN BLACK HOLE ACCRETION
TRANSPORT OF LARGE-SCALE POLOIDAL FLUX IN BLACK HOLE ACCRETION
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DOI：
10.1088/0004-637x/707/1/428
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发表时间：
2009-06
期刊：
影响因子：
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通讯作者：
K. Beckwith;J. Hawley;J. Krolik
中科院分区：
文献类型：
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作者：
K. Beckwith;J. Hawley;J. Krolik
我们报告了一个全球的，三维的GRMHD模拟的吸积环嵌入在一个大尺度的垂直磁场环绕史瓦西黑洞。该模拟研究了湍流吸积盘内大规模垂直场的演变，以及适合发射射流和风的全球磁场结构是否可以发展。我们发现磁通量运动的“日冕机制”主要在圆盘体外运行，主导着全球磁通量演化。在这种机制中，由轨道剪切驱动的磁应力产生了大规模的磁场半环，这些磁场半环径向向内拉伸，然后重新连接，导致磁通量位置的不连续跳跃。相比之下，盘内吸积直接带来的通量很少或根本没有。日冕机制在轨道轴周围的真空漏斗中建立了一个偶极磁场，其磁场强度由内盘的磁压力和气体压力共同调节。这些结果促使我们重新评估先前对与吸积有关的磁通量运动的描述。局部图像被磁通量的内在全局特征削弱了。“有效粘度”与“有效电阻率”相竞争的公式由于磁动力学的非线性和驱动质量运动的相同湍流（传统上被称为“粘度”）可以改变磁拓扑这一事实而受到破坏。
We report on a global, three-dimensional GRMHD simulation of an accretion torus embedded in a large-scale vertical magnetic field orbiting a Schwarzschild black hole. This simulation investigates how a large-scale vertical field evolves within a turbulent accretion disk and whether global magnetic field configurations suitable for launching jets and winds can develop. We find that a “coronal mechanism” of magnetic flux motion, which operates largely outside the disk body, dominates global flux evolution. In this mechanism, magnetic stresses driven by orbital shear create large-scale half-loops of magnetic field that stretch radially inward and then reconnect, leading to discontinuous jumps in the location of magnetic flux. In contrast, little or no flux is brought in directly by accretion within the disk itself. The coronal mechanism establishes a dipole magnetic field in the evacuated funnel around the orbital axis with a field intensity regulated by a combination of the magnetic and gas pressures in the inner disk. These results prompt a re-evaluation of previous descriptions of magnetic flux motion associated with accretion. Local pictures are undercut by the intrinsically global character of magnetic flux. Formulations in terms of an “effective viscosity” competing with an “effective resistivity” are undermined by the nonlinearity of the magnetic dynamics and the fact that the same turbulence driving mass motion (traditionally identified as “viscosity”) can alter magnetic topology.