Interplanetary Diffusion Coefficients for Cosmic Rays

Information on the cosmic-ray diffusion coefficient, κ, derived from near-Earth observations of the solar modulation of galactic electron fluxes and from the near-Earth power spectra of the interplanetary magnetic field, has been used to study the heliocentric radial dependence of κ and to derive limits on the spatial extent of the solar modulation region. Representing κ as a separable function of radius r and rigidity, and assuming κ(r) ∝ r^n, we can place a limit on the power law exponent, n ≾ 1.2. The distance of the modulation boundary is a function of n, and, e.g., for n = 0, falls into the range of 6-25 AU.