[论文解读] Magnetic Fields in Accretion Disks: A Review

Amir Jafari|arXiv (Cornell University)|Apr 21, 2019

Astrophysics and Star Formation Studies参考文献 2被引用 5

一句话总结

本文综述了吸积盘中大尺度磁场向内卷吸的理论模型，重点解决磁场所在盘面中部因扩散而迅速衰减的挑战。研究提出，磁浮力与湍流抽运可将偶极磁场向上输运，减少磁重联，从而在盘面表层维持强而弯曲的磁场，为磁-离心机制高效喷流提供必要条件。

ABSTRACT

We review the current theoretical models of the inward advection of the large scale external magnetic fields in accretion discs. The most plausible theories for launching astrophysical jets rely on strong magnetic fields at the inner parts of the host accretion disks. An internal dynamo can in principle generate small scale magnetic fields in situ but generating a large scale field in a disk seems a difficult task in the dynamo theories. In fact, as far as numerous numerical experiments indicate, a dynamo-generated field in general would not be coherent enough over the large length scales of order the disk's radius. Instead, a large scale poloidal field dragged in from the environment, and compressed by the accretion, provides a more promising possibility. The difficulty in the latter picture, however, arises from the reconnection of the radial field component across the mid-plane which annihilates the field faster than it is dragged inward by the accretion. We review the different mechanisms proposed to overcome these theoretical difficulties. In fact, it turns out, that a combination of different effects, including magnetic buoyancy and turbulent pumping, is responsible for the vertical transport of the field lines toward the surface of the disk. The radial component of the poloidal field vanishes at the mid-plane, which efficiently impedes reconnection, and grows exponentially toward the surface where it can become much larger than the vertical field component. This allows the poloidal field to be efficiently advected to small radii until the allowed bending angle drops to of order unity, and the field can drive a strong outflow.

研究动机与目标

• 解决尽管在盘面中部磁扩散迅速，仍需维持大尺度磁场的理论挑战。
• 评估来自星际介质的外部磁场向内卷吸作为内盘强磁场来源的可行性。
• 研究磁场几何构型——特别是偶极磁场的弯曲角度——如何决定喷流发射效率。
• 分析湍流输运机制在抑制磁重联、维持相干大尺度磁场中的作用。
• 调和数值模拟显示的磁场卷吸现象与磁流体动力学理论预测的快速扩散之间的矛盾。

提出的方法

• 分析向内卷吸速度 $ v_{\text{adv}} \sim \nu / r $ 与向外扩散速度 $ v_{\text{diff}} \sim (\eta / h)(B_r / B_z) $ 的平衡，导出条件 $ B_r / B_z \sim h / r $。
• 考虑盘面中部的磁重联问题：方向相反的径向磁场会迅速湮灭，除非受到抑制。
• 引入磁浮力与湍流抽运作为机制，将磁场线垂直输运，减少盘面中部的磁重联。
• 使用包含湍流扩散系数 $ \eta $ 的磁流体动力学方程建模磁场演化，假设 $ \nu / \eta \sim 1 $。
• 应用 Blandford-Payne 条件 $ \tan^{-1}(B_r / B_z) \geq 30^\circ $ 作为高效喷流发射的基准。
• 评估中平面处 $ B_r \ll B_z $（抑制磁重联）而表面处 $ B_r \gg B_z $（支持喷流）的磁场构型。

实验结果

研究问题

• RQ1 为何动力学理论难以在盘面半径范围内生成大尺度相干磁场？
• RQ2 尽管盘面中部磁重联迅速，大尺度磁场如何能在向内卷吸过程中存活？
• RQ3 何种物理机制可实现磁通量的垂直输运，以减少盘面中部的磁重联并维持磁场相干性？
• RQ4 通过磁-离心机制实现高效喷流所需的最小弯曲角度是多少？
• RQ5 湍流扩散系数与粘性系数如何影响吸积盘中磁场的径向与垂直输运？

主要发现

• 若盘面中部磁重联被抑制，来自星际介质的大尺度磁场可实现向内卷吸。
• 磁浮力与湍流抽运能有效将偶极磁场线向上输运，减少盘面中部的径向磁场分量。
• 径向磁场分量 $ B_r $ 随高度呈指数增长，使 $ B_r / B_z \gg 1 $，满足 $ \geq 30^\circ $ 弯曲角条件。
• 由于中平面处 $ B_r \approx 0 $，磁重联被抑制，从而防止磁场快速湮灭。
• 磁场可被卷吸至小半径，直至弯曲角降至约 1 量级，此时可驱动强喷流。
• 数值模拟支持磁场卷吸，但磁流体动力学理论预测扩散更快，除非引入垂直输运机制。