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

为了研究星系核气盘的形成和活动机制，进行了三维全球磁流体动力学（MHD）模拟。初始状态是一个由弱环向磁场缠绕的气体环面。我们假设气体环面在恒星和暗物质的引力势下旋转。我们发现(1)magnetorotational环驱动磁扰动不稳定增长,(2)星际气体通过麦克斯韦应力下降运输角动量和形式核气盘,(3)星系磁场是放大到1微G,(4)意味着方位磁场的方向逆转期约为1 gyr,和(5)的逆转意味着磁场是由活跃的逃脱的磁通气体磁盘银河晕。我们还对核气体盘进行了全球三维MHD模拟，发现与我们银河系1kpc内观测到的相似的磁环是由气体盘磁通量的浮力上升形成的。数值模拟表明，极向磁通量被带向银河系中心。我们进行了二维和三维的全球电阻MHD模拟，结果表明，质量吸积部分是由射流的角动量提取驱动的。利用三维MHD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，虽然扭结不稳定性在磁力线强烈扭曲的区域增长，但它不会破坏喷流。我们在MHD模拟器CANS （Coordinated Astronomical Numerical Software）上实现了基于Roe方案的三维MHD仿真引擎。CANS的手册和网页已被翻译成英文，并在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中使用。

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.