We investigate the conditions for the presence of a magnetically inactive dead zone in protostellar disks, using 3-D shearing-box MHD calculations including vertical stratification, Ohmic resistivity and time-dependent ionization chemistry. Activity driven by the magnetorotational instability fills the whole thickness of the disk at 5 AU, provided cosmic ray ionization is present, small grains are absent and the gas-phase metal abundance is sufficiently high. At 1 AU the larger column density of 1700 g/cm^2 means the midplane is shielded from ionizing particles and remains magnetorotationally stable even under the most favorable conditions considered. Nevertheless the dead zone is effectively eliminated. Turbulence mixes free charges into the interior as they recombine, leading to a slight coupling of the midplane gas to the magnetic fields. Weak, large-scale radial fields diffuse to the midplane where they are sheared out to produce stronger azimuthal fields. The resulting midplane accretion stresses are just a few times less than in the surface layers on average.Comment: to appear in the Astrophysical Journal; 25 pages, 10 figure

Abstract. The occurrence of the magnetorotational instability (MRI) in vertically stratified galactic disks is considered. Global 3D nonlinear MHD simulations with the ZEUSMP code are performed in a cylindric computational domain. Due to the evolution of the MRI toroidal and poloidal components of the mean magnetic fields are generated. The results are also applied to very young galaxies which are assumed to possess strong magnetic fields already after a few 10 8 years. The dependence of MRI growth rate on the shear strength is shown. The velocity dispersion grows with height and reaches values of about 5 km s −1 in good agreement with observations and close to the predictions of Sellwood & Balbus (1999). For strong magnetic fields the MRI is suppressed but it is not suppressed by turbulence initially present in the disk.

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