Angular Distributions of Solar Energetic Particles

Abstract

We report observations, using data from the Wind spacecraft, of temporal variations in the angular distributions of H, He, O, and Fe ions at MeV energies during solar energetic particle (SEP) events under a variety of conditions. Detailed time-dependent angular distributions of O and Fe during SEP events are reported for the first time. Extended periods of particle streaming in small gradual events are consistent with continued acceleration that is expected at shock waves driven from the Sun by coronal mass ejections (CMEs). Particles accelerated from SEP events of moderate size show especially strong streaming along the magnetic fields inside an old CME. For the largest events, however, streaming rapidly diminishes, showing behavior that is qualitatively consistent with the theory of Ng et al. In very intense events, that theory predicts rapid growth of proton-generated Alfvén waves, even near and beyond 1 AU, that soon scatter and isotropize the particles. We show, for the first time, cases where SEP streaming is organized by the value of the plasma beta βp, the ratio of the proton thermal energy to magnetic energy. Higher initial turbulence in the high-βp plasma may require less additional wave growth to reach significant levels of scattering in the largest SEP events.