Focused transport of energetic particles along magnetic field lines draped around a coronal mass ejection

Abstract

The transport of particles in interplanetary space can be dominated by adiabatic focusing of particle pitch angle distributions under conditions when the ratio of the particle scattering mean free path to the magnetic field focusing length, λ/L, is ∼1. Since for the average Archimedean spiral field configuration, L ∼ 1 AU at Earth orbit, the only reported events dominated by focusing are those nearly “scatter‐free” events where λ ∼ 1 AU. However, if the interplanetary magnetic field is distorted so that the focusing length L is small, then even if the interplanetary scattering mean free path were small, focused transport would be expected to dominate, since λ/L would still be ∼1. We present evidence for such a event, obtained with observations of ∼0.1–1 MeV nucleon−1 protons and alpha particles obtained with the University of Maryland/Max‐Planck‐Institut experiment on the ISEE 3 spacecraft during the decay phase of the June 6, 1979, solar particle event. During the early portion of the decay phase of this event, interplanetary magnetic field lines were apparently draped around a coronal mass ejection, leading to a small focusing length L on the western flank where ISEE 3 was located. We observed a period of very slow decrease of particle intensity, along with large sunward anisotropy in the solar wind frame, which are inconsistent with predictions of the standard Fokker‐Planck equation models for diffusive transport; however, we are able to fit the observations assuming that focused transport dominates and that the particle pitch angle scattering is isotropic.