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Cosmic rays and magnetic fields are important non-thermal components of the ISM of spiral galaxies. Cosmic ray electrons in the presence of the ISM magnetic fields give rise to synchrotron emission, which is one of the most powerful probes of magnetic fields in galaxies. Interpretation of synchrotron intensity 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 density. We examine the energy equipartition assumption between cosmic rays and magnetic fields using both: test-particle simulations (important to capture the effect of the magnetic field structure) and MHD simulations by considering cosmic rays as an additional diffusive fluid (important to consider the effects of gas pressure). 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 strength are statistically independent. Nevertheless, small-scale cosmic ray structures are abundant at low energies. These are particles trapped in random magnetic bottles. These conclusions can significantly change the interpretation of synchrotron observations and thus our understanding of strength and structure of magnetic fields in the Milkyway and nearby spiral galaxies. |
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