  We study the radial ionization structure at the surface of an X-ray
illuminated accretion disk. We plot the expected iron K$\alpha$ line energy as
a function of the Eddington ratio and of the distance of the emitting matter
from the central source, for a non-rotating and a maximally-rotating black
hole. We compare the predicted disk line energies with those measured in an
archival sample of active galactic nuclei observed with {\it Chandra}, {\it
XMM-Newton} and {\it Suzaku}, and discuss whether the line energies are
consistent with the radial distances inferred from reverberation studies. We
also suggest using rapidly-variable iron K$\alpha$ lines to estimate the
viscosity parameter of an accretion disk. There is a forbidden region in the
line energy versus Eddington ratio plane, at low Eddington ratios, where an
accretion disk cannot produce highly-ionized iron K$\alpha$ lines. If such
emission is observed in low-Eddington-ratio sources, it is either coming from a
highly-ionized outflow, or is a blue-shifted component from fast-moving neutral
matter.
