  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.
