Cross-field transport of solar energetic particles in a large-scale fluctuating magnetic field

Abstract

The trajectories of solar energetic particles in an interplanetary magnetic field exhibiting large-scale fluctuations due to footpoint motions originating in the photosphere are simulated using a full-orbit test-particle code. The cross-field transport experienced by the particles in three propagation conditions (scatter-free, with scattering mean free path λ = 0.3 AU and λ = 2 AU) is characterized in the Parker spiral geometry. The role of expansion of the magnetic field with radial distance from the Sun is taken into consideration in the calculation of particle displacements and diffusion coefficients from the output of the simulations. It is found that transport across the magnetic field is enhanced in the λ = 0.3 AU and λ = 2 AU cases, compared to the scatter-free case. Values of the ratios of perpendicular to parallel diffusion coefficients vary between 0.01 and 0.08. The ratio of latitudinal to longitudinal diffusion coefficient perpendicular to the magnetic field is typically 0.2, suggesting that transport in latitude may be less efficient.