Enhancement of Turbulent Viscosity by Global Magnetic Fields in Accretion Disks

Extract

A model of magnetohydrodynamic (MHD) turbulence in accretion disks with global magnetic fields is constructed using a second-order closure modeling of turbulence. The transport equations of the Reynolds stress tensor, the Maxwell stress tensor, and the cross-helicity tensor (the correlation of velocity fluctuation and magnetic fluctuation) are closed by second-order quantities using a two-scale direct interaction approximation (TSDIA). The quantities appearing these equations are considered to be those averaged in the vertical direction of the disks. The turbulence is assumed to be stationary. We are interested only in the effects of the global magnetic fields on the turbulence in the disks, i.e., no dynamo processes are considered, and the global magnetic fields are supposed to be embedded in the disk a priori.