  The vertical structure of optically thick accretion disks is investigated in
the two-zone approximation. The disk is divided into an underlying disk and a
corona, where the latter is defined as the upper surface layers for which the
effective optical depth is unity. It is found that a significant part of the
accretion flow (or dissipation rate) can take place in the corona if the scale
height of the magnetic field is larger than that of the disk. The presence of
such a dissipative corona leads to a modification in the topology of local disk
solutions. For example, these solutions are found from local stability analysis
to be both secularly and thermally stable, for accretion rates which are a
factor $\approx$ four higher than those inferred from the stability of standard
disk solutions. Thus, the applicability of optically thick disks with
dissipative coronas are not as restrictive as disks without such coronas and
can provide an attractive explanation for the origin of the soft spectral
component observed in black hole X-ray binary systems.
