Why is the cosmic-ray ionization rate in diffuse clouds so high?
In plain words
Molecules such as H3+ show that diffuse interstellar clouds are ionized about ten times faster than the cosmic rays measured near the Sun can explain. Either unseen low-energy cosmic rays exist or the chemistry is misunderstood.
Precise statement
H3+ absorption gives a mean primary ionization rate per $\mathrm{H_2}$ molecule $\zeta_2 \sim 3.5 \times 10^{-16}\,\mathrm{s}^{-1}$ in diffuse molecular clouds (Indriolo and McCall 2012), more than 10 times the rate implied by the local interstellar cosmic-ray spectrum measured by Voyager 1. Determine the cause: a spatially variable low-energy (1 to 100 MeV) cosmic-ray population, nearby sources, or errors in the H3+ chemistry.
What would settle it
Ionization rates along many sightlines with distances, compared with propagation models of low-energy cosmic rays from identified sources.
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