Time & Place: Thursday, 3:00pm, August 27th, Lecture Hall, 3rd floor   

Abstract: The classical alpha disk model has been successfully used to model a wide range of accretion disks. However, there are still many puzzles when the standard disk model is used to explain the observations. After magneto-rotational instability (MRI) is realized to be the physical mechanism for angular momentum transfer and dissipation in the disks, we are able to understand the properties of the disks without the alpha assumption. I will describe results from recent shearing box and global radiation MHD simulations to understand the thermal stability and formation of corona in black hole accretion disks. I will also describe important new effects caused by magnetic field that do not exist in the classical alpha disk model, which will help us understand many puzzles when the classical alpha disk model is compared with observations.