Time: July 9th, 2:00pm

Speaker: Xuening Bai (Princeton University)

Location: 3rd floow, middle conference room

Title: Magnetorotational Instability: Successes and Limitations in Shearing-Box Simulations

Abstract:

The magnetorotational instability (MRI) is believed to be the primary mechanism for driving accretion in a wide range of astrophysical disks. The non-linear saturation of the MRI is best studied by shearing-box simulations, which represents a local patch of a thin accretion disk. I will begin by summarizing four types of shearing-box simulations,
depending on whether vertical density stratification and a net vertical magnetic flux are included. These simulations demonstrate the complexity of the MRI turbulence, and also reveal issues of numerical convergence. The most realistic but least studied type of shearing-box simulations is the case with both vertical stratification and vertical net flux. I will describe our most recent simulations of this type, with midplane plasma beta (ratio of gas to magnetic pressure) for the net vertical field ranging from 10^4 to 100. These simulations demonstrate the critical dependence of the properties of MRI turbulence on the vertical net flux: stronger outflow/disk wind is launched with increasing net flux. Although the fate of the outflow can not be determined from shearing-box simulations, we speculate that it is unlikely to be connected to a magneto-centrifugal wind until mass loss evacuates the disk to the extent so that beta<1.