Hall Effect-Mediated Magnetic Flux Transport in Protoplanetary Disks
Hall Effect-Mediated Magnetic Flux Transport in Protoplanetary Disks
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DOI：
10.3847/1538-4357/836/1/46
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发表时间：
2017-02-10
影响因子：
4.9
通讯作者：
Stone, James M.
中科院分区：
文献类型：
--
作者：
Bai, Xue-Ning;Stone, James M.
最近的研究表明，原行星盘（PPD）的全球演化在很大程度上取决于穿过盘的极向磁通量的大小。磁通量的数量也必须与磁盘共同演变，作为磁通量传输的结果，这是一个知之甚少的过程。在弱电离气体中，如在PPD中，磁通量大部分被冻结在电子流体中，除非电阻率很大。当磁盘主要是层流，我们表明，电子和离子之间的相对漂移（霍尔漂移），和离子和中性流体（双极漂移）可以发挥主导作用的磁通量的运输。使用二维模拟，将霍尔效应和双极扩散（AD）与规定的扩散率，我们表明，当大规模的极向场与磁盘旋转对齐，霍尔效应迅速拖动磁通向内的中平面区域，而它慢慢地推动磁通向外上方/下方的中平面。这导致了一个高度径向拉长场配置作为霍尔剪切不稳定性的全球表现。该场配置进一步促进AD在中平面处的快速向外通量传输，导致不稳定性饱和。在准稳定状态下，磁通量在所有高度以近似相同的速率向外传输，并且该速率与无霍尔情况相当。对于反对齐场极性，霍尔效应一致地向外传输磁通量，导致中平面区域中的大部分垂直场配置。上层的磁力线首先径向向内弯曲，然后向外发射盘状风。总的来说，向外通量传输的净速率大约是对齐情况下的两倍。此外，磁通量传输率随磁盘磁化强度的增加而增加。绝对输运率对磁盘微物理敏感，这仍有待于在未来的研究中探索。
The global evolution of protoplanetary disks (PPDs) has recently been shown to be largely controlled by the amount of poloidal magnetic flux threading the disk. The amount of magnetic flux must also coevolve with the disk, as a result of magnetic flux transport, a process that is poorly understood. In weakly ionized gas as in PPDs, magnetic flux is largely frozen in the electron fluid, except when resistivity is large. When the disk is largely laminar, we show that the relative drift between the electrons and ions (the Hall drift), and the ions and neutral fluids (ambipolar drift) can play a dominant role on the transport of magnetic flux. Using two-dimensional simulations that incorporate the Hall effect and ambipolar diffusion (AD) with prescribed diffusivities, we show that when large-scale poloidal field is aligned with disk rotation, the Hall effect rapidly drags magnetic flux inward at the midplane region, while it slowly pushes flux outward above/below the midplane. This leads to a highly radially elongated field configuration as a global manifestation of the Hall-shear instability. This field configuration further promotes rapid outward flux transport by AD at the midplane, leading to instability saturation. In quasi-steady state, magnetic flux is transported outward at approximately the same rate at all heights, and the rate is comparable to the Hall-free case. For anti-aligned field polarity, the Hall effect consistently transports magnetic flux outward, leading to a largely vertical field configuration in the midplane region. The field lines in the upper layer first bend radially inward and then outward to launch a disk wind. Overall, the net rate of outward flux transport is about twice as fast as that of the aligned case. In addition, the rate of flux transport increases with increasing disk magnetization. The absolute rate of transport is sensitive to disk microphysics, which remains to be explored in future studies.