论文标题
磁化不稳定性的饱和度和磁升高的积聚盘的起源
Saturation of the magnetorotational instability and the origin of magnetically elevated accretion discs
论文作者
论文摘要
我们提出，通过净垂直磁场螺纹的积聚盘中角动量转运的强度取决于自我调节机制：磁化不稳定性（MRI）的增长，直到其自身的湍流电阻率抑制了圆盘厚度尺度上最快的生长模式。鉴于先前模拟证据支持MRI衍生湍流的结构的弱假设，提出的机制再现了粘性$α$ - 参数的已知缩放，$α\ proptoβ_z^{ - 1/2} $。在这里，$β_z=8πp_g/b_ {z0}^2 $是圆盘中平面上的初始等离子体$β$ - 参数，$ b_ {z0} $是净场，$ p_g $是中等平面气体压力。我们将论点推广到具有强肌外环磁场的光盘，其中MRI生长速率是根据弱场极限改变的。如果要在磁性上升高，则需要其他湍流来源，并且在模拟中看到的中平面附近的尺度高度增加。我们推测，与有效电阻率扩大的电流板相关的撕裂模式可能是高架盘中湍流增强的可能来源。
We propose that the strength of angular momentum transport in accretion discs threaded by net vertical magnetic field is determined by a self-regulation mechanism: the magnetorotational instability (MRI) grows until its own turbulent resistivity damps the fastest growing mode on the scale of the disc thickness. Given weak assumptions as to the structure of MRI-derived turbulence, supported by prior simulation evidence, the proposed mechanism reproduces the known scaling of the viscous $α$-parameter, $α\propto β_z^{-1/2}$. Here, $β_z = 8πp_g/B_{z0}^2$ is the initial plasma $β$-parameter on the disc midplane, $B_{z0}$ is the net field, and $p_g $ is the midplane gas pressure. We generalize the argument to discs with strong suprathermal toroidal magnetic fields, where the MRI growth rate is modified from the weak-field limit. Additional sources of turbulence are required if such discs are to become magnetically elevated, with the increased scale heights near the midplane that are seen in simulations. We speculate that tearing modes, associated with current sheets broadened by the effective resistivity, are a possible source of enhanced turbulence in elevated discs.
