Modulation of low‐energy cosmic rays

Recent theories have related the diffusion of cosmic rays in the solar wind to the power spectrum of interplanetary magnetic field fluctuations. In this study the power spectrum‐diffusion coefficient relation for low‐energy protons ( T < 80 MeV) is tested directly, and the relative contribution to the scattering from directional discontinuities in the magnetic field is examined. Modulation parameters V w /K rr , the solar wind speed divided by the radial diffusion coefficient, are calculated from Pioneer 6 magnetic field and solar wind data and are compared with simultaneous Imp 3 observations of the 20‐ to 80‐MeV proton flux. Daily variations in the 40‐ to 80‐MeV proton flux are found to respond to changes in V w /K rr , as is predicted by a perturbation solution of the Fokker‐Planck equation once the contribution from directional discontinuities in the magnetic field is subtracted from the diffusion coefficients. This finding is interpreted to imply that a large percentage of the identified discontinuities are basically tangential. A lack of correlation between the 20‐ to 40‐MeV flux and the parameter changes is interpreted as being due to either the invalidity of the theory at lower energies or the presence of a continuous flux of high‐energy solar protons. A well‐defined time lag between changes in the 60‐ to 80‐MeV proton flux and corresponding changes in V w /K rr is observed during a period when the interplanetary magnetic field is predominantly radial and the Imp 3‐sun‐Pioneer 6 angle is greater than 10°. This lag is consistent with the interpretation of little or no cross‐field diffusion or a perpendicular diffusion coefficient less than 3 × 10 19 cm²/s.