The cosmic ray ionization rate in the Galactic disk, as determined from
  observations of molecular ions
  We have obtained estimates for the cosmic-ray ionization rate (CRIR) in the
Galactic disk, using a detailed model for the physics and chemistry of diffuse
interstellar gas clouds to interpret previously-published measurements of the
abundance of four molecular ions: ArH$^+$, OH$^+$, H$_2$O$^+$ and H$_3^+$. For
diffuse $atomic$ clouds at Galactocentric distances in the range $R_g \sim 4 -
9$ kpc, observations of ArH$^+$, OH$^+$, and H$_2$O$^+$ imply a mean primary
CRIR of $(2.2 \pm 0.3) \exp [(R_0-R_g)/4.7\,\rm{kpc}] \times 10^{-16} \rm \,
s^{-1}$ per hydrogen atom, where $R_0=8.5$ kpc. Within diffuse $molecular$
clouds observed toward stars in the solar neighborhood, measurements of H$_3^+$
and H$_2$ imply a primary CRIR of $(2.3 \pm 0.6) \times 10^{-16}\,\,\rm s^{-1}$
per H atom, corresponding to a total ionization rate per H$_2$ molecule of
$(5.3 \pm 1.1) \times 10^{-16}\,\,\rm s^{-1},$ in good accord with previous
estimates. These estimates are also in good agreement with a rederivation,
presented here, of the CRIR implied by recent observations of carbon and
hydrogen radio recombination lines along the sight-line to Cas A. Here, our
best-fit estimate for the primary CRIR is $2.9 \times 10^{-16}\,\,\rm s^{-1}$
per H atom. Our results show marginal evidence that the CRIR in diffuse
molecular clouds decreases with cloud extinction, $A_{\rm V}({\rm tot})$, with
a best-fit dependence $\propto A_{\rm V}({\rm tot})^{-1}$ for $A_{\rm V}({\rm
tot}) \ge 0.5$.
