Local Magnetohydrodynamic Models of Layered Accretion Disks

Using numerical MHD simulations, we have studied the evolution of themagnetorotational instability in stratified accretion disks in which theionization fraction (and therefore resistivity) varies substantially withheight. This model is appropriate to dense, cold disks around protostars ordwarf nova systems which are ionized by external irradiation of cosmic rays orhigh-energy photons. We find the growth and saturation of the MRI occurs onlyin the upper layers of the disk where the magnetic Reynolds number exceeds acritical value; in the midplane the disk remains queiscent. The verticalPoynting flux into the "dead", central zone is small, however velocityfluctuations in the dead zone driven by the turbulence in the active layersgenerate a significant Reynolds stress in the midplane. When normalized by thethermal pressure, the Reynolds stress in the midplane never drops below about10% of the value of the Maxwell stress in the active layers, even though theMaxwell stress in the dead zone may be orders of magnitude smaller than this.Significant mass mixing occurs between the dead zone and active layers.Fluctuations in the magnetic energy in the active layers can drive verticaloscillations of the disk in models where the ratio of the column density in thedead zone to that in the active layers is <10. These results have importantimplications for the global evolution of a layered disk, in particular theremay be residual mass inflow in the dead layer. We discuss the effects that dustin the disk may have on our results.Comment: Accepted by Ap.