2012ApJ...745...91I

Observations of H⁺₃ in the Galactic diffuse interstellar medium have led to various surprising results, including the conclusion that the cosmic-ray ionization rate (ζ₂) is about one order of magnitude larger than previously thought. The present survey expands the sample of diffuse cloud sight lines with H⁺₃ 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 < ζ₂< (10.6±8.2)x10^–16/s with a mean value of ζ₂= (3.5^+5.3_–3.0)x10^–16/s. Upper limits (3σ) derived from non-detections of H⁺₃ are as low as ζ₂ < 0.4x10^–16/s. These low upper limits, in combination with the wide range of inferred cosmic-ray ionization rates, indicate variations in ζ₂ between different diffuse cloud sight lines. A study of ζ₂versus 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 ζ₂ 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.