Vol. 619
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.