Global multifluid simulations of the magnetorotational instability in   radially stratified protoplanetary disks
  The redistribution of angular momentum is a long standing problem in our understanding of protoplanetary disk (PPD) evolution. The magnetorotational instability (MRI) is considered a likely mechanism. We present the results of a study involving multifluid global simulations including Ohmic dissipation, ambipolar diffusion and the Hall effect in a dynamic, self-consistent way. We focus on the turbulence resulting from the non-linear development of the MRI in radially stratified PPDs and compare with ideal MHD simulations.   In the multifluid simulations the disk is initially set up to transition from a weak Hall dominated regime, where the Hall effect is the dominant non-ideal effect but approximately the same as or weaker than the inductive term, to a strong Hall dominated regime, where the Hall effect dominates the inductive term. As the simulations progress a substantial portion of the disk develops into a weak Hall dominated disk. We find a transition from turbulent to laminar flow in the inner regions of the disk, but without any corresponding overall density feature.   We introduce a dimensionless parameter, $\alpha_\mathrm{RM}$, to characterise accretion with $\alpha_\mathrm{RM} \gtrsim 0.1$ corresponding to turbulent transport. We calculate the eddy turnover time, $t_\mathrm{eddy}$, and compared this with an effective recombination timescale, $t_\mathrm{rcb}$, to determine whether the presence of turbulence necessitates non-equilibrium ionisation calculations. We find that $t_\mathrm{rcb}$ is typically around three orders of magnitude smaller than $t_\mathrm{eddy}$. Also, the ionisation fraction does not vary appreciably. These two results suggest that these multifluid simulations should be comparable to single fluid non-ideal simulations. 
Donna Rodgers-Lee, Tom P. Ray, Turlough P. Downes
astro-ph.SR astro-ph.EP