Cosmic-ray perpendicular diffusion coefficient computed using Voyager data at 15 AU

Abstract

Perpendicular diffusion due to either field line wandering or random gradient and curvature effects makes a substantial contribution to the radial transport of particles at distances of about 5 AU and beyond when lines of the interplanetary magnetic field (IMF) become almost azimuthal. Here test particle trajectories are followed in a field model which uses a magnetic field sample taken by Voyager 2 at 14.8 AU. Techniques previously developed (Moussas et al., 1982a) are employed to calculate the perpendicular diffusion coefficient for 100 MeV protons and arising from random gradient and curvature effects. The result K⊥=(2.8±0.9)×10²¹ cm² s⁻¹ shows a substantial increase above the value determined previously at 5 AU and, assuming a power law radial dependence K∼r ^β, implies β ≃ 0.8–1.1. This result is consistent with observations of the cosmic-ray radial gradient.