The diffusion coefficient describing the motion of cosmic-ray particles in the interplanetary magnetic field is determined from the power spectrum of magnetic irregularities observed on space probes, and is found to be proportional to $R\beta$ ($R=\mathrm{particle}\mathrm{magnetic}\mathrm{rigidity}$, $\beta =\mathrm{particle}\mathrm{velocity}$). Near minimum solar activity, only this dependence of the diffusion coefficient on $R$ and $\beta$ can account for the observed long-term intensity variations of cosmic-ray protons and helium nuclei down to 10 MeV per nucleon in energy. Thus, the motion of cosmic rays in interplanetary space may be quantitatively related to the observed magnetic field.

• Received 13 June 1966

DOI:https://doi.org/10.1103/PhysRevLett.17.203