In section 6. Interstellar and circumstellar matter
Production of atomic hydrogen by cosmic rays in dark clouds
Most of the hydrogen in dark clouds is in the form of H2, but small amounts of atomic hydrogen are known to be present at different depths. In the darkest parts of clouds, where the interstellar UV photons are extincted, cosmic rays (CR) dominate the process of maintaining a small fraction of atomic hydrogen through the destruction of H2. This paper presents a model of the attenuation of interstellar CR and its effect in dissociating H2 using the latest data from the Voyager 1 spacecraft. From their work, the authors conclude that the dominant effect of the CR is through secondary electrons produced during the primary CR ionization process. These secondary electrons represent the only source of atomic hydrogen at column densities larger than about 1021 cm-2.