Investigating the Cosmic-Ray Ionization Rate in the Galactic Diffuse
  Interstellar Medium through Observations of H3+
  Observations of H3+ in the Galactic diffuse interstellar medium (ISM) have
led to various surprising results, including the conclusion that the cosmic-ray
ionization rate (zeta_2) is about 1 order of magnitude larger than previously
thought. The present survey expands the sample of diffuse cloud sight lines
with H3+ observations to 50, with detections in 21 of those. Ionization rates
inferred from these observations are in the range (1.7+-1.3)x10^-16
s^-1<zeta_2<(10.6+-8.2)x10^-16 s^-1 with a mean value of
zeta_2=(3.5^+5.3_-3.0)x10^-16 s^-1. Upper limits (3 sigma) derived from
non-detections of H3+ are as low as zeta_2<0.4x10^-16 s^-1. These low
upper-limits, in combination with the wide range of inferred cosmic-ray
ionization rates, indicate variations in zeta_2 between different diffuse cloud
sight lines. A study of zeta_2 versus N_H (total hydrogen column density) shows
that the two parameters are not correlated for diffuse molecular cloud sight
lines, but that the ionization rate decreases when N_H increases to values
typical of dense molecular clouds. Both the difference in ionization rates
between diffuse and dense clouds and the variation of zeta_2 among diffuse
cloud sight lines are likely the result of particle propagation effects. The
lower ionization rate in dense clouds is due to the inability of low-energy
(few MeV) protons to penetrate such regions, while the ionization rate in
diffuse clouds is controlled by the proximity of the observed cloud to a site
of particle acceleration.
