On energy equipartition between cosmic rays and magnetic fields

TL;DR

This study challenges the common assumption of energy equipartition between cosmic rays and magnetic fields, showing they are statistically independent at turbulent scales through simulations, affecting interpretation of synchrotron data.

Contribution

It provides the first detailed simulation-based evidence that cosmic ray and magnetic field energies are uncorrelated, revising previous equipartition assumptions.

Findings

No spatial correlation between cosmic rays and magnetic fields at turbulent scales.

Cosmic ray density and magnetic energy are statistically independent.

Cosmic rays are highly inhomogeneous, especially at low energies.

Abstract

Interpretations of synchrotron observations often assume a tight correlation between magnetic and cosmic ray energy densities. We examine this assumption using both test-particle simulations of cosmic rays and MHD simulations which include cosmic rays as a diffusive fluid. We find no spatial correlation between the cosmic rays and magnetic field energy densities at turbulent scales. Moreover, the cosmic ray number density and magnetic field energy density are statistically independent. Nevertheless, the cosmic ray spatial distribution is highly inhomogeneous, especially at low energies because the particles are trapped between random magnetic mirrors. These results can significantly change the interpretation of synchrotron observations and thus our understanding of the strength and structure of magnetic fields in the Milky Way and nearby spiral galaxies.