A semiempirical approach to low-energy cosmic ray propagation in the diffuse interstellar medium

Context. We investigate the ionization of the diffuse interstellar medium by cosmic rays by modeling their propagation along the wandering magnetic fields using a Monte Carlo method. We explore how particle trapping and second-order Fermi processes affect the ionization of the medium. Aims. We study how low-energy comic rays propagate in turbulent, translucent molecular clouds, and how they regulate the ionization and both lose and gain energy from the medium. Methods. As a test case, we used high spatial resolution (0.03 pc) CO maps of a well-studied high latitude translucent cloud, MBM 3, to model turbulence. The propagation problem is solved with a modified Monte Carlo procedure that includes trapping, energization, and ionization losses. Results. In the homogeneous medium, trapping and re-energization do not produce a significant effect. In the nonuniform medium, particles can be trapped for a long time inside the cloud. This modifies the cosmic ray distribution due to stochastic acceleration at the highest energies (similar to 100 MeV). At lower energies, the re-energization is too weak to produce an appreciable effect. The change in the energy distribution does not significantly affect the ionization losses, so ionization changes are due to trapping effects. Conclusions. Our Monte Carlo approach to cosmic ray propagation is an alternative method for solving the transport equation. This approach can be benchmarked to gas observations of molecular clouds. Using this approach, we demonstrate that stochastic Fermi acceleration and particle trapping occurs in inhomogeneous clouds, significantly enhancing their ionization.