Magnetically-driven wind and non-equilibrium ionisation in protoplanetary disks
The magnetorotational instability (MRI) is important for the dynamical evolution of protoplanetary disks through accretion due to magnetic stress and mass loss due to the disk winds. Near the disk midplane, the timescale of chemical reactions governing MHD effects is much smaller than the dynamical timescale, so we can use the equilibrium ionization degree for MHD simulations. In the surface layer, however, the chemical timescale could be longer than the dynamical timescale. Thus, it is important to consider the non-equilibrium ionization degree.
We implemented our calculation scheme for time-dependent ionization degree into the Athena MHD code and performed three dimensional local shearing box simulations. When the disk has mostly dissipated, we find that the ionization degree in the surface layer becomes smaller than the equilibrium solution. This is because the poorly ionized gas in the lower layers is transferred to the surface layers by the disk wind. This effect may be important in the MHD and chemical evolution of protoplanetary disks.