Anisotropy of shock‐accelerated ion distributions in interplanetary space

Abstract

Recent reports of energetic particles upstream of interplanetary shocks have concentrated upon the ability of the shocks to accelerate a small percentage of the solar wind thermal ion population. In keeping with the diffusive shock acceleration model, Lee (1983) derived a theory which predicts the asymptotic steady state approached by the energetic particle distribution and low‐frequency wave spectra upstream of interplanetary traveling shocks. More recently, Kennel et al. (1986) have performed a detailed test of the Lee theory by applying it to the observations of the November 12, 1978 shock viewed by the ISEE spacecraft. A major point of departure between the theory and observations rests with the spatial dependence of the particle intensity and anisotropy in the upstream region. We argue that this discrepancy is due to the presence of solar flare particles. These particles form an additional seed population which alters the upstream boundary condition of the energetic population. The resulting anisotropy of the energetic particle distribution several scale lengths upstream of the shock is proportional to the ratio of the streaming of the shock‐accelerated population to the density of the solar flare population. Comparison of this theoretical result with the observed upstream anisotropy suggests that the measured upstream anisotropies are due largely to the streaming of the solar flare ion population. We also compare this theory to the measured particle intensities and anisotropies in the April 4–5, 1979 shock event reported by Sanderson et al. (1985b) and conclude that very little of the upstream particle anisotropy measured for this event can be attributed to the shock acceleration process. Since energetic storm particle (ESP) events at 1 AU are frequently observed in association with solar flare particles, it is anticipated that the discussion presented here should describe a commonly observed aspect of ESP events.