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

TL;DR

This paper models low-energy cosmic ray propagation in the turbulent interstellar medium using a Monte Carlo approach, revealing how trapping and re-energization affect ionization and energy distribution in molecular clouds.

Contribution

It introduces a semiempirical Monte Carlo method to study cosmic ray propagation, including trapping and energization effects in turbulent molecular clouds.

Findings

Trapping significantly alters cosmic ray distribution in nonuniform media.

Re-energization has minimal impact at energies below 100 MeV.

Ionization changes are mainly due to trapping effects, not re-energization.

Abstract

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 study how low-energy cosmic rays propagate in turbulent, translucent molecular clouds, and how they regulate the ionization and both lose and gain energy from the medium. As a test case, we use 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. In a homogeneous medium, trapping and re-energization do not produce a significant effect. In a 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 (about 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.