Cosmic-ray propagation in molecular clouds

TL;DR

This paper investigates how cosmic rays penetrate molecular clouds, affecting ionisation rates, by considering cosmic-ray spectra, magnetic effects, and cloud properties, revealing that magnetic mirroring significantly reduces ionisation.

Contribution

It provides a detailed calculation of cosmic-ray ionisation rates in molecular clouds, incorporating magnetic focusing and mirroring effects, and clarifies the impact of low-energy cosmic rays.

Findings

Magnetic mirroring reduces ionisation rates by a factor of 3-4.

Low-energy cosmic rays are responsible for ionisation in clouds.

Attenuation of cosmic-ray flux depends on magnetic field configuration.

Abstract

Cosmic-rays constitute the main ionising and heating agent in dense, starless, molecular cloud cores. We reexamine the physical quantities necessary to determine the cosmic-ray ionisation rate (especially the cosmic ray spectrum at E < 1 GeV and the ionisation cross sections), and calculate the ionisation rate as a function of the column density of molecular hydrogen. Available data support the existence of a low-energy component (below about 100 MeV) of cosmic-ray electrons or protons responsible for the ionisation of diffuse and dense clouds. We also compute the attenuation of the cosmic-ray flux rate in a cloud core taking into account magnetic focusing and magnetic mirroring, following the propagation of cosmic rays along flux tubes enclosing different amount of mass and mass-to-flux ratios. We find that mirroring always dominates over focusing, implying a reduction of the cosmic-ray ionisation rate by a factor of 3-4 depending on the position inside the core and the magnetisation of the core.