Magnetic Mirroring and Focusing of Cosmic Rays

TL;DR

This paper investigates how magnetic mirroring and focusing influence cosmic ray ionization in dense astrophysical environments, revealing conditions where these effects cancel or lead to significant ionization variations.

Contribution

It provides analytical expressions to accurately calculate cosmic ray ionization rates considering magnetic field configurations, especially in magnetic pockets.

Findings

Magnetic effects can cancel each other out in certain conditions.

Ionization rates are mainly controlled by energy loss attenuation in uniform magnetic fields.

Magnetic pockets can drastically reduce cosmic ray ionization.

Abstract

We study the combined impact of magnetic mirroring and focusing on the ionization by cosmic rays (CRs) in dense molecular clouds and circumstellar disks. We show that for effective column densities of up to $\sim10^{25}$ cm$^{-2}$ (where ionization is the main mechanism of energy losses by CRs) the two effects practically cancel each other out, provided the magnetic field strength has a single peak along field lines. In this case the ionization rate at a given location is controlled solely by attenuation of interstellar CRs due to energy losses. The situation is very different in the presence of magnetic pockets -- local minima of the field strength, where the CR density and thus ionization can be reduced drastically. We obtain simple analytical expressions allowing accurate calculation of the ionization rate in these regions.