  We demonstrate that the observed increase of some nebular line ratios with
height above the midplane in the diffuse ionized gas (DIG) in the Milky Way and
other galaxies is a natural consequence of the progressive hardening of the
radiation field far from the midplane ionizing sources. To obtain increasing
temperatures and line ratios away from the midplane, our photoionization
simulations of a multi-component interstellar medium do not require as much
additional heating (over and above that from photoionization) as previous
studies that employed one dimensional, spherically averaged models.
  Radiation leaking into the DIG from density bounded H II regions is generally
harder in the H-ionizing continuum and has its He-ionizing photons suppressed
compared to the ionizing source of the H II region. In line with other recent
investigations, we find that such leaky H II region models can provide elevated
temperatures and line ratios, and a lower He+ fraction in the DIG. For a
composite model representing the relative spectral types of O stars in the
solar neighbourhood, we find that additional heating less than 1.0E-26 n_e
ergs/s/cm^3 can reproduce the observed elevated line ratios in the DIG. This
additional heating is considerably less than previous estimates due to the
natural hardening of the radiation field reaching large heights in our
simulations.
