Investigating the cosmic-ray ionization rate in the Galactic diffuse interstellar medium through observations of H+3

Nick Indriolo, Benjamin J. McCall

Research output: Contribution to journalArticlepeer-review


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) × 10 -16s-1 < ζ2 < (10.6 ± 8.2) × 10-16s-1 with a mean value of ζ2 = (3.5+5.3 -3.0) × 10-16s-1. Upper limits (3σ) derived from non-detections of H+ 3 are as low as ζ2 < 0.4 × 10 -16s-1. 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 ζ2 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 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.

Original languageEnglish (US)
Article number91
JournalAstrophysical Journal
Issue number1
StatePublished - Jan 20 2012
Externally publishedYes


  • cosmic rays

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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