The Formation and Structure of a Strongly Magnetized Corona above Weakly Magnetized Accretion Disks

We use three-dimensional magnetohydrodynamical (MHD) simulations to study the formation of a corona above an initially weakly magnetized, isothermal accretion disk. We also describe a modification to time-explicit numerical algorithms for MHD which enables us to evolve highly stratified disks for many orbital times. We find that MHD turbulence driven by the magnetorotational instability (MRI) produces strong amplification of weak fields within two scale heights of the disk midplane in a few orbital times. About 25 % of the magnetic energy generated by the MRI within two scale heights escapes due to buoyancy, producing a strongly magnetized corona above the disk. Most of the buoyantly rising magnetic energy is dissipated between 3 and 5 scale heights, suggesting the corona will also be hot. The average vertical disk structure consists of a weakly magnetized turbulent core below a strongly magnetized corona which is stable to the MRI. The largescale field structure in both the disk and corona is toroidal. The functional form of the stress is flat within two scale heights, but proportional to the density above two scale heights. For initially weak uniform vertical fields, we find the exponential growth of magnetic field via axisymmetric vertical modes of the MRI produces strongly buoyant sheets of magnetic energy which break the disk apart into horizontal channels. These channels rise several scale heights vertically before the onset of the Parker instability distorts the sheets and allows matter to flow back towards the midplane and reform a disk. We suggest this evolution may be relevant to the dynamical processes which disrupt the inner regions of a disk when it interacts with a strongly magnetized central object.