  Observations of ionized AGN outflows have provided compelling evidence that
the radiation field transfers both momentum and energy to the plasma. At parsec
scale distances in AGN, energy transfer can dominate, in which case the only
force needed to launch an outflow is that from gas pressure. Much closer to the
black hole, gravity dominates thermal energy due to insufficient heating by the
radiation and the gas is in the so-called `cold' regime. Only magnetic or
radiation forces can then lead to outflow, but it is unclear at what
temperature and ionization state the radiation force weakens, as these
properties depend on the spectral energy distribution (SED). In this work, we
survey the parameter space of radiation forces due to spectral lines resulting
from blackbody SEDs with varying temperatures in the range $\sim 10^4 - 10^6$~K
to identify the radiation temperature at which line-driving begins to lose
efficiency. We find that the temperature $\lesssim4\times10^5$~K marks the
transition to inefficient line driving. We also self-consistently compute the
heating and cooling balance to estimate the gas temperature, so that our
parameter survey covers the transition where thermal driving goes from
negligible to comparable to line driving. We summarize a large set of
hydrodynamical simulations of radial flows to illustrate how the wind
properties change during the transition and the dependence of these properties
on the assumed SED and governing flow parameters.
