  Using numerical MHD simulations, we have studied the evolution of the
magnetorotational instability in stratified accretion disks in which the
ionization fraction (and therefore resistivity) varies substantially with
height. This model is appropriate to dense, cold disks around protostars or
dwarf nova systems which are ionized by external irradiation of cosmic rays or
high-energy photons. We find the growth and saturation of the MRI occurs only
in the upper layers of the disk where the magnetic Reynolds number exceeds a
critical value; in the midplane the disk remains queiscent. The vertical
Poynting flux into the "dead", central zone is small, however velocity
fluctuations in the dead zone driven by the turbulence in the active layers
generate a significant Reynolds stress in the midplane. When normalized by the
thermal pressure, the Reynolds stress in the midplane never drops below about
10% of the value of the Maxwell stress in the active layers, even though the
Maxwell 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 vertical
oscillations of the disk in models where the ratio of the column density in the
dead zone to that in the active layers is <10. These results have important
implications for the global evolution of a layered disk, in particular there
may be residual mass inflow in the dead layer. We discuss the effects that dust
in the disk may have on our results.
