The Fractional Ionization of the Warm Neutral Interstellar Medium
  When the neutral interstellar medium is exposed to EUV and soft X ray
radiation, the argon atoms in it are far more susceptible to being ionized than
the hydrogen atoms. We make use of this fact to determine the level of
ionization in the nearby, warm, neutral medium (WNM). By analyzing FUSE
observations of ultraviolet spectra of 44 hot subdwarf stars a few hundred pc
away from the Sun, we can compare column densities of Ar I to those of O I,
where the relative ionization of oxygen can be used as a proxy for that of
hydrogen. The measured deficiency [ Ar I/O I]= 0.427+/- 0.11dex below the
expectation for a fully neutral medium implies that the electron density n(e)
\sim 0.04cm^- 3 if n(H)=0.5cm^- 3. This amount of ionization is considerably
larger than what we expect from primary photoionizations resulting from cosmic
rays, the diffuse X-ray background, and X-ray emitting sources within the
medium, along with the additional ionizations caused by energetic secondary
photoelectrons, Auger electrons, and photons from helium recombinations. We
favor an explanation that bursts of radiation created by previous, nearby
supernova remnants that have faded by now may have elevated the ionization, and
the gas has not yet recombined to a quiescent level. A different alternative is
that the low energy portion of the soft X ray background is poorly shielded by
the H I because it is frothy and has internal pockets of very hot, X-ray
emitting gases.
