  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.
