  The iron K line is perhaps the most important spectral diagnostic available
in the study of accreting black holes. The line is thought to result from the
reprocessing of external X-rays by the surface of the accretion disk. However,
as is observed in the solar corona, illumination by energetic particles may
also produce line emission. In principle, such a process may be uncorrelated
with the observed X-rays and could explain some of the unexpected variability
behavior of the Fe line. This paper compares predictions of iron K flux
generated by impacting electrons and protons to that from photoionization.
Non-thermal power-laws of electrons are considered as well as thermal
distributions of electrons and virialized protons. The electrons are thought to
originate in a magnetically dominated accretion disk corona, while the protons
are considered in the context of a two phase (hot/cold) accretion scenario. In
each case, the Fe K flux from particle impact is found to be < 1% of that
produced by photoionization by a hard X-ray power-law (normalized to the same
energy flux as the particles). Thus, the electrons or protons must strike the
disk with 100--10,000 times more energy flux than radiation for particle impact
to be a significant producer of Fe K flux. This situation is difficult to
reconcile with the observations of hard X-ray spectra, or the proposed particle
acceleration mechanisms in the accretion disk corona. Truncated accretion flows
must be externally illuminated by hard X-rays in order to produce the Fe line,
as proton impact is very inefficient in generating line emission. In contrast
to the Sun, our conclusion is that, with the possible exception for localized
regions around magnetic footpoints, particle impact will not be an important
contributor to the X-ray emission in accreting black holes.
