Why is the distribution of Cosmic Rays isotropic?

We observe that cosmic rays (CRs) are isotropic on the sky, i.e. they come from all directions with no apparent preference.

I understand this is related to the deflection of these CRs by the magnetic fields in the Galaxy - the magnetic field causes the CRs to spiral, with a gyroradius $R$.

This gyroradius is very large, of $\sim$ parsec order of magnitude.

My question is why does this large gyroradius imply that the distribution of CRs should be isotropic?

Thanks

• Imagine you are more than 10 gyroradii away from a source of energetic particles (emitted in a spherically symmetric way... ignore how for now) and the only field is a quasi-static magnetic field, $\mathbf{B}_{o}$. Would you expect to see any significant fluxes of particles moving perpendicular to $\mathbf{B}_{o}$? Or would you expect to only get particles moving along $\mathbf{B}_{o}$ since they do not suffer deviations from the Lorentz force? – honeste_vivere Apr 15 '16 at 20:19

Polarisation measurements in the spiral arms indicate that the magnetic fields there are pretty tangled rather than nicely aligned. So the direction of motion of cosmic rays becomes isotropic on essentially the same distance scale as the scale of the 'tangles', at least for cosmics rays of low enough energy to have $\approx 1$pc gyroradius. Directions will be randomised on a distance scale comparable to the scale of magnetic field fluctuations or the gyroradius, whichever is larger.