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Observations of H3+ in the Galactic diffuse interstellar medium (ISM) haveled to various surprising results, including the conclusion that the cosmic-rayionization rate (zeta_2) is about 1 order of magnitude larger than previouslythought. The present survey expands the sample of diffuse cloud sight lineswith H3+ observations to 50, with detections in 21 of those. Ionization ratesinferred from these observations are in the range (1.7+-1.3)x10^-16s^-1<zeta_2<(10.6+-8.2)x10^-16 s^-1 with a mean value ofzeta_2=(3.5^+5.3_-3.0)x10^-16 s^-1. Upper limits (3 sigma) derived fromnon-detections of H3+ are as low as zeta_2<0.4x10^-16 s^-1. These lowupper-limits, in combination with the wide range of inferred cosmic-rayionization rates, indicate variations in zeta_2 between different diffuse cloudsight lines. A study of zeta_2 versus N_H (total hydrogen column density) showsthat the two parameters are not correlated for diffuse molecular cloud sightlines, but that the ionization rate decreases when N_H increases to valuestypical of dense molecular clouds. Both the difference in ionization ratesbetween diffuse and dense clouds and the variation of zeta_2 among diffusecloud sight lines are likely the result of particle propagation effects. Thelower ionization rate in dense clouds is due to the inability of low-energy(few MeV) protons to penetrate such regions, while the ionization rate indiffuse clouds is controlled by the proximity of the observed cloud to a siteof particle acceleration.

Investigating the Cosmic-Ray Ionization Rate in the Galactic Diffuse Interstellar Medium through Observations of H3+