由Parker不稳定性驱动发电机介导的磁化Keplerian盘中的高吸积率
High accretion rates in magnetised Keplerian discs mediated by a Parker instability driven dynamo
摘要：
吸积盘中的磁流体应力在过去的15年里一直是理论研究的热点。大多数圆盘模拟都假设了一个小的初始磁场，并研究了由磁旋转不稳定性引起的湍流。然而，在星系核和一些双星系统中的气体盘可能有显着的初始磁化。受此启发，我们进行了理想的磁流体动力学模拟强磁化，垂直分层光盘在开普勒潜在的。我们的初始平衡位形具有与热压力相等的方位磁场，对Parker不稳定性是不稳定的。这导致锚定在盘的中平面中的磁场弧从中平面排出到大约五个刻度高度。过渡到湍流发生主要是通过磁旋转的不稳定性，在所产生的垂直领域，虽然磁旋转剪切不稳定性在未扰动的方位场发挥了重要的作用，以及，特别是在中平面浮力弱。高磁应力和流体动力应力的出现，产生一个有效的$α$ -值约为0.1，在我们的最高分辨率运行。由磁浮力从盘排出的方位磁场通过径向场的拉伸而连续地补充，径向场是当气体包裹沿着倾斜的磁场线在线性重力场中滑动时产生的。这个发电机过程，其中帕克不稳定性导致的场线弯曲导致方位场的重新创建，意味着高度磁化的盘在天体物理上是可行的，并且它们具有高吸积率。
英文摘要：
Hydromagnetic stresses in accretion discs have been the subject of intense theoretical research over the past one and a half decades. Most of the disc simulations have assumed a small initial magnetic field and studied the turbulence that arises from the magnetorotational instability. However, gaseous discs in galactic nuclei and in some binary systems are likely to have significant initial magnetisation. Motivated by this, we performed ideal magnetohydrodynamic simulations of strongly magnetised, vertically stratified discs in a Keplerian potential. Our initial equilibrium configuration, which has an azimuthal magnetic field in equipartion with thermal pressure, is unstable to the Parker instability. This leads to the expelling of magnetic field arcs, anchored in the midplane of the disc, to around five scale heights from the midplane. Transition to turbulence happens primarily through magnetorotational instability in the resulting vertical fields, although magnetorotational shear instability in the unperturbed azimuthal field plays a significant role as well, especially in the midplane where buoyancy is weak. High magnetic and hydrodynamical stresses arise, yielding an effective $α$-value of around 0.1 in our highest resolution run. Azimuthal magnetic field expelled by magnetic buoyancy from the disc is continuously replenished by the stretching of a radial field created as gas parcels slide in the linear gravity field along inclined magnetic field lines. This dynamo process, where the bending of field lines by the Parker instability leads to re-creation of the azimuthal field, implies that highly magnetised discs are astrophysically viable and that they have high accretion rates.
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