Magnetorotational and Magnetic Buoyancy Instabilities in Differentially Rotating Disks

1.通过三维局部磁流体动力学（MHD）模拟，研究了重力分层差转盘中磁旋不稳定性和磁浮力不稳定性的非线性演化。我们发现磁吸积盘有两种状态；气体压力占主导的准稳态和磁压力占主导的准稳态。在后一种状态下，发生浮力驱动的磁场放大。在大尺度垂直磁场作用下，对差分旋转环面进行了MHD模拟，研究了射流速度和质量流出率与磁场强度的关系。通过三维MHD模拟，我们发现环面非轴对称不稳定性的增长也会在射流内部产生非轴对称结构。研究了弱环面磁场作用下环面的时间演化。环面内部磁场的放大在10个旋转周期后达到饱和状态，此时发生了磁湍流。在这种状态下，环面气体压力与磁压力之比约为10。由于有效的角动量输运，角动量分布接近于开普勒旋转。角动量输运率α的有效值在0.01 ~ 0.1之间。我们还发现了丝状磁压占优区的出现和磁环浮力上升的现象。我们对中心恒星的偶极磁场与周围吸积盘之间的相互作用进行了轴对称二维模拟。从磁盘注入的磁螺旋度驱动热等离子体的磁重联和抛射。我们将这一机制应用于在原恒星中观测到的x射线耀斑和光学喷流。我们为各种并行计算机编写了并行化的MHD代码，并证实了代码的效率几乎与处理元素的数量成正比。

1.By means of three-dimensional local magnetohydrodynamic (MHD) simulations, we studied the nonlinear evolution of magnetorotational instabilities and magnetic buoyancy instabilities in gravitationa11y stratified, differentially rotating disks. We found that magnetic accretion disks have two states ; a gas pressure dominated quasi-steady state and a magnetic pressure dominated state. In the latter state, buoyancy driven magnetic field amplification occurs.2.We carried out global MHD simulations of differentially rotating torus threaded by large scale vertical magnetic fields and studied the dependence of jet speed and mass outflow rates on the magnetic field strength. By 3D MHD simulations, we showed that the growth of non-axisymmetric instabilities in the torus also create non-axisymmetric structures inside the jet.3.We studied the time evolution of a torus threaded by weak toroidal magnetic fields. The amplification of magnetic fields inside the torus saturates after 10 rotation period when magnetic turbulence develops. The ratio of the gas pressure to magnetic pressure in the torus is about 10 in this state. Because of the effcient angular momentum transport, the angular momentum distribution approaches to the Keplerian rotation. The effective value of the angular momentum transport rate, alpha, is between 0.01 and 0.1. We also found that filamentary magnetic pressure dominated regions appear and that magnetic loops rise buoyantly.4.We carried out axisymmetric 2D simulations of the interaction between the dipole magnetic field of the central star and surrounding accretion disks. The magnetic helicity injected from the disk drives magnetic reconnection and ejection of hot plasmoids. We applied this mechanism to X-ray flares and optical jets observed in protostars.5.We wrote parallelized MHD codes for various parallel computers and confirmed that the efficiency of the code is almost proportional to the number of processing elements.