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 H2 molecule $\zeta_2 \sim 3.5e-16\,\mathrm{s}^{-1}$ 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.