Ionized Gas in the First 10 Kiloparsecs of the Interstellar Galactic
  Halo: Metal Ion Fractions
  We present direct measures of the ionization fractions of several sulfur ions
in the Galactic warm ionized medium (WIM). We obtained high resolution
ultraviolet absorption line spectroscopy of post-asymptotic giant branch stars
in the the globular clusters Messier 3 [(l,b)=(42.2, +78.7); d=10.2 kpc, z=10.0
kpc] and Messier 5 [(l,b)=(3.9, +46.8); d=7.5 kpc, z = +5.3 kpc] with the
Hubble Space Telescope and Far Ultraviolet Spectroscopic Explorer to measure,
or place limits on, the column densities of S I, S II, S III, S IV, S VI, and H
I. These clusters also house millisecond pulsars, whose dispersion measures
give an electron column density from which we infer the H II column in these
directions. We find fractions of S+2 in the WIM for the M 3 and M 5 sight lines
x(S+2) = N(S+2)/N(S) = 0.33+/-0.07 and 0.47+/-0.09, respectively, with
variations perhaps related to location. With negligible quantities of the
higher ionization states, we conclude S+ and S+2 account for all of the S in
the WIM. We extend the methodology to study the ion fractions in the warm and
hot ionized gas of the Milky Way, including the high ions Si+3, C+3, N+4, and
O+5. The vast majority of the Galactic ionized gas is warm (T ~ 10^4 K) and
photoionized (the WIM) or very hot (T > 4x10^5 K) and collisionally ionized.
The common tracer of ionized gas beyond the Milky Way, O+5, traces <1% of the
total ionized gas mass of the Milky Way.
