  Magnetohydrodynamic accretion disk simulations suggest that much of the
energy liberated by the magnetorotational instability (MRI) can be channeled
into large-scale toroidal magnetic fields through dynamo action. Under certain
conditions, this field can dominate over gas and radiation pressure in
providing vertical support against gravity, even close to the midplane. Using a
simple model for the creation of this field, its buoyant rise, and its coupling
to the gas, we show how disks could be driven into this magnetically dominated
state and deduce the resulting vertical pressure and density profiles. Applying
an established criterion for MRI to operate in the presence of a toroidal
field, we show that magnetically supported disks can have two distinct
MRI-active regions, separated by a "dead zone" where local MRI is suppressed,
but where magnetic energy continues to flow upward from the dynamo region
below. We suggest that the relative strengths of the MRI zones, and the local
poloidal flux, determine the spectral states of X-ray binaries. Specifically,
"intermediate" and "hard" accretion states occur when MRI is triggered in the
hot, upper zone of the corona, while disks in "soft" states do not develop the
upper MRI zone. We discuss the conditions under which various transitions
should take place and speculate on the relationship of dynamo activity to the
various types of quasi-periodic oscillations that sometimes appear in the hard
spectral components. The model also explains why luminous accretion disks in
the "soft" state show no signs of the thermal/viscous instability predicted by
standard alpha models.
