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

用三维全球磁流体(MHD)模拟研究了银河核气盘的形成和活动机制。初始状态是一个被弱环向磁场缠绕的气体环面。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)环面中不断增长的磁致不稳定驱动着磁场的湍流；(2)星际气体通过麦克斯韦应力传递角动量而坠落，形成核气盘；(3)银河系平均磁场被放大到1微G；(4)平均方位向磁场的方向以约1 Gyr的周期反转；(5)平均磁场的反转是由磁通量从气盘到银晕的浮力逃逸所驱动的。我们还对核气盘进行了全球3DMHD模拟，发现类似于在银河系1kpc内观察到的磁环是由气盘的磁通量浮力上升形成的。数值模拟表明，极向磁通量被带向银河系中心。我们对垂直磁场穿透圆盘的喷流形成进行了二维和三维整体阻性MHD模拟，结果表明质量吸积部分是由喷流的角动量提取所驱动的。用三维MHD程序研究了直角坐标系下磁力驱动射流的稳定性。我们发现，尽管扭结不稳定性在磁力线强烈扭曲的区域发展，但它并不会扰乱喷流。我们在磁流体模拟器CANS上实现了基于ROE格式的三维MHD模拟引擎。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冬季数值天体物理学校。

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.