  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.
