Characteristics of sunward flowing proton and alpha particle fluxes of moderate intensity

Abstract

The diffusive streaming of 1.3‐ to 2.3‐MeV per nucleon protons has been found to be predominantly toward the sun during periods between prompt solar particle events. This sunward streaming occurs for essentially all proton intensities from <0.012 to 1.2 (cm² s sr MeV)−1 and for all solar wind velocities. The average radial component (16% ± 3%) of the diffusive anisotropy of 1.4‐ to 2.4‐MeV per nucleon alpha particles is very similar to that observed for protons (14% ± 1%), a finding suggesting a common origin. These periods are characterized by a limited variance in the proton intensities (10−1.2±1.1 cm−2 s−1 sr−1 MeV−1), in the proton spectra (E−3.0±0.8), and in the α/p ratio (3% ± 2%). The sunward diffusion of protons and alpha particles indicates that a positive radial gradient is characteristic of these modestly enhanced fluxes. A steady state propagation model which includes adiabatic energy loss and a source of particles beyond 1 AU produces the average radial anisotropy and its dependence on the solar wind velocity for κrr ∼ 4 × 1020 cm² s−1. The direction of the diffusive anisotropy is strongly dependent on the magnetic field direction, a situation indicating κ⊥ < κ∥. However, the two directions are not identical, a condition indicating nonnegligible flow perpendicular to the average field direction when averaged over a 6‐hour interval.