Large-scale azimuthal structures of turbulence in accretion disks: Dynamo triggered variability of accretion

Abstract

We investigate the significance of large-scale azimuthal, magnetic, and velocity modes for the magnetorotational instability (MRI) turbulence in accretion disks. We perform three-dimensional global ideal MHD simulations of global stratified protoplanetary disk models. Our domains span azimuthal angles of π/4, π/2, π, and 2π. We observe up to 100% stronger magnetic fields and stronger turbulence for the restricted azimuthal domain models π/2 and π/4 compared to the full 2π model. We show that for those models the Maxwell stress is larger due to strong axisymmetric magnetic fields generated by the αΩ dynamo. Large radial extended axisymmetric toroidal fields trigger temporal magnification of accretion stress. All models display a positive dynamo-α in the northern hemisphere (upper disk). The parity is distinct in each model and changes on timescales of 40 local orbits. In model 2π, the toroidal field is mostly antisymmetric with respect to the midplane. The eddies of the MRI turbulence are highly anisotropic. The major wavelengths of the turbulent velocity and magnetic fields are between one and two disk scale heights. At the midplane, we find magnetic tilt angles around 8°-9° increasing up to 12°-13° in the corona. We conclude that an azimuthal extent of π is sufficient to reproduce most turbulent properties in three-dimensional global stratified simulations of magnetized accretion disks. © 2012 The American Astronomical Society. All rights reserved.