Relative distribution of cosmic rays and magnetic fields

TL;DR

This study uses simulations to show that cosmic ray density and magnetic field energy are statistically independent at galactic scales, challenging the common assumption of their tight correlation in interpreting synchrotron data.

Contribution

The paper demonstrates through simulations that cosmic ray distribution is uncorrelated with magnetic field energy, questioning the equipartition assumption used in astrophysical analyses.

Findings

Cosmic ray density and magnetic field energy are statistically independent.

Low-energy cosmic rays can be trapped by magnetic mirrors.

No tight correlation exists between cosmic rays and magnetic fields at relevant scales.

Abstract

Synchrotron radiation from cosmic rays is a key observational probe of the galactic magnetic field. Interpreting synchrotron emission data requires knowledge of the cosmic ray number density, which is often assumed to be in energy equipartition (or otherwise tightly correlated) with the magnetic field energy. However, there is no compelling observational or theoretical reason to expect such tight correlation to hold across all scales. We use test particle simulations, tracing the propagation of charged particles (protons) through a random magnetic field, to study the cosmic ray distribution at scales comparable to the correlation scale of the turbulent flow in the interstellar medium ($\simeq 100\,{\rm pc}$ in spiral galaxies). In these simulations, we find that there is no spatial correlation between the cosmic ray number density and the magnetic field energy density. In fact, their distributions are approximately statistically independent. We find that low-energy cosmic rays can become trapped between magnetic mirrors, whose location depends more on the structure of the field lines than on the field strength.