Ulysses observations of solar energetic particles to high heliographic latitudes

Abstract

At the time of the flare on the Bastille Day of 2000, Ulysses spacecraft was at 3.17 AU from the sun, high heliographic latitude of 62° South, and 116° in longitude east of the Earth. The event produced large fluxes of energetic particles up to energies >100 MeV at both Ulysses and the Earth. Enhancements of energetic particles were immediately observed at the Earth, their onset times consistent with the velocity dispersion due to the streaming of particles along magnetic field lines from the CME shock in the corona to the Earth. To the contrary, at Ulysses, the energetic particles from the solar event were not detected until 4-11 hours later, and the increases of particle intensity were much more gradual. The onset times of particles at Ulysses were not organized by particle speed; rather they depended on both particle rigidity and speed. Model analyses using the focused transport theory and a simple diffusion model indicate that the particles seen by Ulysses were injected around the time of peak flaring at 1024UT and that the particle transport to Ulysses requires much smaller mean free path than from the sun to the Earth. Unless the magnetic turbulence could cause such slow particle transport at high latitudes, the observations suggest that Ulysses was not directly connected by magnetic field lines to the shock driven by the CME. Energetic particles reached the latitude at Ulysses by transport across nominal Parker magnetic field lines. Such efficient latitudinal transport may be indicative of random motion of magnetic field line in the solar corona that causes braided magnetic field lines in the heliosphere.