The ionization of the neutral gas in an interstellar molecular cloud plays a key role in the cloud’s evolution, helping to regulate the heating and cooling processes, the chemistry and molecule formation, and coupling the gas to magnetic fields. Usually starlight provides this ultraviolet radiation, but it is mostly restricted to localized regions near massive stars. For the bulk of the neutral gas in the Milky Way, ionization is governed by low energy cosmic-rays (CRs), fast-moving protons or atomic nuclei. Direct observations from the Earth can only probe high energy CRs because the solar wind restricts the penetration of CR into the solar system, but in the past few decades, the total CR ionization rate has been estimated indirectly with observations of diagnostic molecules and ions. Those values, however, rely on some uncertain estimates like the abundances of secondary species, gas densities, the rates of chemical reactions and not least, the amount of the dominant molecular species, molecular hydrogen.
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Source: Phys.org