Mean free paths and diffusion coefficients for energetic protons at small heliodistances calculated using Helios 1 and 2 data

Abstract

Pitch angle scattering of energetic particles (100 MeV) in the interplanetary medium are studied using Helios 1 and 2 magnetometer and plasma data during 1976 near the minimum of solar activity. An IMF configuration was used in the computer experiments which allowed the pitch angle diffusion coefficient, Dμμ and hence the parallel mean free path, λ∥ to be determined. The radial mean free path was found to vary as λr ∼ r-0.9 between 0.4 and 1 AU, but between 0.3 and 0.4 AU it decreases significantly. To reconcile our value of λr at 1 AU, lying between 0.01 and 0.02 AU, with the average prompt solar proton event profile, an increasing value of λr at lower radial distances would be required.