An observational test for correlations between cosmic rays and magnetic fields

We derive the magnitude of fluctuations in total synchrotron intensity in the Milky Way, both from observations and from theory under various assumption about the relation between cosmic rays and interstellar magnetic fields. Given the relativ e magnitude of the fluctuations in the Galactic magnetic field suggested by the Faraday rotatio n and polarization data, the observations are inconsistent with local energy equipartiti on between cosmic rays and magnetic fields. Our analysis of synchrotron fluctuations suggests th at the distribution of cosmic rays is nearly uniform at scales of order 100pc, in contrast to that of the interstellar magnetic field. A conservative upper limit on the relative variations in the cosmic ray number density is 0.2‐0.4 at the scales of order 100 pc. Our results are consist ent with a mild anticorrelation between cosmic-ray and magnetic energy densities at these scales. Energy equipartition between cosmic rays and magnetic fields still may hold, but at scales e xceeding 1 kpc.