Radial Variation of Magnetic Fluctuations and the Cosmic-Ray Diffusion Tensor in the Solar Wind

The radial evolution of the power spectra of fluctuations in the interplanetary magnetic field is studied under the assumptions that the fluctuations are frozen-in magnetic fluctuations or that they are AlIven waves with wave vector parallel to the average field. These are used to obtain the radial variation of the cosmic-ray diffusion tensor, under the assumption that the tensor is determined principally by P1(k11). It is found that the coefficient for radial diffusion, K , does not increase with r at large distances from the Sun under either assumption concerning the fluctuations. It is concluded that if cosmic-ray scattering has an effective boundary at roughly 5-10 a.u. heliocentric radius, some additional damping mechanism may come into play. It is suggested that ionization of interstellar neutral hydrogen can cause strong damping of waves at about S a.u. heliocentric radius. Subject headings: cosmic rays - hydromagnetics - interplanetary medium - solar wind