Research on the Formation and Activity of Galactic Nuclear Gas Disks Including Magnetohydrodynamical Processes

利用三维全球磁流体动力学（MHD）模拟研究了银河系核气盘的形成和活动机制。初始状态是一个由弱环向磁场穿过的气体环。我们假设气体环在恒星和暗物质的引力势中旋转。我们发现：（1）磁旋转不稳定性的增长驱动磁湍流，（2）星际气体通过麦克斯韦应力输运角动量而在福尔斯中下落，形成核气盘，（3）银河系平均磁场被放大到1 μ G，（4）平均方位磁场的方向反转，周期约为1 Gyr，（5）平均磁场的反转是由磁通量从气体盘向星系晕的浮力逃逸驱动的。我们还对核气盘进行了全球3D MHD模拟，发现了类似于我们星系1kpc内部观测到的磁环，这些磁环是由气盘中的磁通量浮力上升形成的，数值模拟表明极向磁通量被带到星系中心。我们进行了2D和3D的全球电阻MHD模拟射流形成磁盘线程垂直磁场和质量吸积部分驱动的角动量提取射流。利用三维磁流体动力学程序，在直角坐标系下研究了磁驱动射流的稳定性。我们发现，虽然扭结不稳定性增长的磁场线强烈扭曲的区域，它并没有中断射流。我们实现了三维MHD模拟引擎的Roe计划的MHD模拟器CANS（协调天文数值软件）。CANS的手册和网页已被翻译成英文，并用于2006年3月13日至17日在千叶大学举行的亚洲数值天体物理冬季学校。

Three-dimensional global magnetohydrodynamic (MHD) simulations have been carried out to study the mechanism of the formation and activity of galactic nuclear gas disks. The initial state is a gas torus threaded by weak toroidal magnetic fields. We assumed that the gas torus is rotating in the gravitational potential of stars and the dark matter. We found (1) magnetorotational instability growing in the torus drives magnetic turbulence, (2) interstellar gas in falls by transporting angular momentum through Maxwell stress and forms a nuclear gas disk, (3) galactic mean magnetic fields are amplified up to 1 micro G, (4) the direction of mean azimuthal magnetic fields reverses with period of about 1Gyr, and (5) the reversal of mean magnetic fields is driven by the buoyant escape of magnetic flux from the gas disk to the galactic halo. We also carried out global 3D MHD simulations of the nuclear gas disk and found that magnetic loops similar to those observed inside 1kpc of our galaxy are formed by the buoyant rise of the magnetic flux from the gas disk.Numerical simulations indicated that poloidal magnetic flux is carried toward the galactic center. We performed 2D and 3D global resistive MHD simulations of jet formation from disks threaded by vertical magnetic fields and showed that mass accretion is partly driven by the angular momentum extraction by jets. The stability of magnetically driven jets has been studied by using 3D MHD code in Cartesian coordinates. We found that although the kink instability grows in regions where magnetic field lines are strongly twisted, it does not disrupt the jet.We implemented 3D MHD simulation engine based on the Roe scheme to the MHD simulator CANS (Coordinated Astronomical Numerical Software). The manuals and web-pages of CANS have been translated into English and used in the Asian Winter School on Numerical Astrophysics, held in March 13-17, 2006, at Chiba University.