# Diffusive Versus Free-Streaming Cosmic Ray Transport in Molecular Clouds

## TL;DR

This paper compares diffusive and free-streaming models for cosmic ray transport in molecular clouds, highlighting their impact on ionization rates and applying the models to Voyager 1 data, favoring diffusion.

## Contribution

It provides a detailed analysis of conditions for cosmic ray diffusion in molecular clouds and compares the resulting ionization rates with free-streaming models.

## Key findings

- Diffusive propagation better explains Voyager 1 spectrum.
- Different models yield significantly different ionization rates.
- Conditions for CR diffusion depend on magnetic turbulence levels.

## Abstract

Understanding the cosmic ray (CR) ionization rate is crucial in order to simulate the dynamics of, and interpret the chemical species observed in molecular clouds. Calculating the CR ionization rate requires both accurate knowledge of the spectrum of MeV to GeV protons at the edge of the cloud as well as a model for the propagation of CRs into molecular clouds. Some models for the propagation of CRs in molecular clouds assume the CRs to stream freely along magnetic field lines, while in others they propagate diffusively due to resonant scattering off of magnetic disturbances excited by MHD turbulence present in the medium. We discuss the conditions under which CR diffusion can operate in a molecular cloud, calculate the local CR spectrum and ionization rate in both a free-streaming and diffusive propagation model, and highlight the different results from the two models. We also apply these two models to the propagation through the ISM to obtain the spectrum seen by Voyager 1, and show that such a spectrum favors a diffusive propagation model.