Influence of the initial magnetic field topology on the evolution of MHD instabilities in accretion disks

Accretion disks are common objects in universe and various phenomena in disks have been observed. They are thought to originate from MHD instabilities, especially the magneto-rotational instability (MRI) and/or the Parker instability. The MRI causes a turbulent state and amplifies the magnetic field in a disk [e.g., Balbus and Hawley, 1991; Hawley et al., 1995]. The MRI induces the angular momentum transport and dynamo effect in accretion disks and coagulation of dust grain in protoplanetary disks is also presumed. On the other hand, the Parker instability leads to gas outflow from disk surface, and is expected to play a major role in disk evolution [Suzuki et al., 2010]. Moreover, three-dimensional MHD simulation studies revealed complicated time evolution of the system, due to the interaction between the MRI and the Parker instability [e.g., Miller and Stone, 2000]. Thus, it is crucial to clarify the time evolution of MHD instabilities in disks for understanding the accretion disk physics. According to recent simulation studies, it is expected that initial magnetic field topology has a crucial effect on the time evolution of the system. For example, in an unstratified disk simulation, where density and pressure are uniform in the simulation domain, Hawley et al. [1995] showed that turbulence stress and magnetic energy in a purely poloidal filed case are two orders of magnitude greater than those of a purely azimuthal field case. Moreover, in a stratified disk model, where the poloidal component of gravitational acceleration by the central object is taken into consideration, and the density and pressure profiles have poloidal gradients to balance against the gravitational fields, Miller and Stone [2000] revealed the time evolution of the system, such as the alternation of density profile, the vertical motion of magnetic field lines, and the amplification of magnetic energy, are entirely different between purely poloidal and purely azimuthal field situations. Then, to clarify the dependence of time evolution of system on the initial magnetic field topology, we perform three-dimensional MHD simulations in which the magnetic field has both poloidal and azimuthal components, and its spatially averaged quantity is nonzero. As a result, when the poloidal component of the initial magnetic field has nonzero value, in spite of the existence of the azimuthal field that has a lager value than the poloidal component, the MRI can grow in the mid-plane region of the disk, and cause the amplification of magnetic fields. Resultant turbulent stress of the system is slightly greater than that obtained from observation studies. In addition, the Parker instability is observed to be activated by the MRI. Such behaviors are similar to the results of purely poloidal simulations of Miller and Stone [2000] rather than the results where the initial field has both poloidal and azimuthal components conducted by Johansen et al. [2008]. In this presentation, we show these simulation results and discuss the effects of the initial magnetic field topology on the development of MHD turbulence in disks, through the comparison of our results with previous works.