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Observations of the "Warm Ionized Medium" (or, equivalently, the "DiffuseIonized Gas") of the local ISM, the Perseus arm in the Milky Way, and also inseveral other galaxies show strong [NII]6563 (~H-alpha in some cases) and[SII]6717/[NII]6583 = 0.6 - 0.7 in all locations and objects. Other line ratios(e.g., [O III]5007/H-beta) vary considerably. Simple photoionization modelsreproduce the observed spectra, providing extra heating beyond that supplied byphotoionization is assumed (Reynolds, Haffner, & Tufte 1999). With observedgas-phase abundances (not solar), the line ratios in the local arm at b = 0 degare fitted with no extra heating and (S/H) = 13 ppm (solar is 20 ppm). Localgas observed at b = -35 deg requires extra heating of about gamma = 0.75, wheregamma is the extra heating in units of 10^{-25} erg H^{-1} s^{-1}. In thePerseus arm, there are similar results, with a domposition consistent with theGalactic abundance gradient. The requirements for NGC 891 are similar to thePerseus arm: little or no extra heating at |z| = 1 kpc and gamma 3 at 2 kpc. InNGC 891 there is also an increase of 5007/H-alpha with |z| that can only comeabout if most of the ionizing radiation is supplied by stars with T~50000 K.Either their radiation must propagate from the plane to high |z| through verylittle intervening matter, or else the stars are located at high |z|. The totalpower requirement of the extra heating is <15% of the photoionization power.[O~II]3727/H-beta can serve as a useful diagnostic of extra heating, but[S~III] 9065,9531/H-alpha is not useful in this regard.Comment: 32 pages, including 2 figures. To appear in November 20 Ap

The Warm Ionized Medium in the Milky Way and Other Galaxies