  We discuss the local physical changes at the surface of an AGN accretion disk
after the onset of a magnetic flare. The X-ray irradiation by a flare creates a
hot spot at the disk surface where the plasma both heats up and expands in the
vertical direction in order to regain the hydrostatic equilibrium. Assuming
that the magnetic loop causing the flare is anchored deeply within the disk
interior, we derive analytical estimates for the vertical dimension H_hot and
the optical depth tau_es of the heated atmosphere as a function of the position
within the spot. We perform computations for various values of the accretion
rate dm/dt, the fraction f_cor of radiation dissipated within the disk corona,
and the covering factor f_cover of the disk surface with flare-illuminated
patches. It turns out that generally we can distinguish three characteristic
radial zones within the disk showing a qualitatively different behavior of the
heated material. In the innermost regions of the disk (inner zone) the
expansion of the disk material is restricted by strong gravitational forces.
Further out, the flare source, initially above the disk, soon becomes embedded
by the expanding disk atmosphere. At these intermediate disk radii (middle
zone) the material is optically thick thus greatly modifying the observed
radiation by multiple Compton scattering. We show exemplary spectra models
obtained from Monte Carlo simulations illustrating the trends. In the outermost
regions of the disk (outer zone) the expanding material is optically thin and
its influence on the observed spectra is smaller but pressure gradients in
radial directions should cause the development of a fountain-like dynamical
structure around the flare source. We discuss the observational consequences of
our results.
