Saturday, November 26, 2016

Global multifluid simulations of the magnetorotational instability in radially stratified protoplanetary disks


Rodgers-Lee et al


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, αRM, to characterise accretion with αRM ≳ 0.1 corresponding to turbulent transport. We calculate the eddy turnover time, teddy, and compared this with an effective recombination timescale, trcb, to determine whether the presence of turbulence necessitates non-equilibrium ionisation calculations. We find that trcb is typically around three orders of magnitude smaller than teddy. 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.

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