At the time of the solar flare on the Bastille Day of2000, the Ulysses spacecraft was at 3.17 AU from the Sun, 62° south in heliographic latitude, and 116° in longitude east of the Earth. Solar wind and magnetic field measurements by Ulysses indicate that the coronal mass ejection (CME) of this event had a limited size in both latitude and longitude, although it was a halo CME as seen in the Solar and Heliospheric Observatory coronagraph images. 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, with their onset times consistent with the velocity dispersion due to the streaming of particles along magnetic field lines from the location of particle acceleration 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 in different energy channels were not organized by particle speed; rather they depended on both particle rigidity and speed, indicating that the transport of particles to Ulysses at high latitudes had a diffusive nature. The first-order anisotropy in the 40-90 MeV proton flux was significantly larger than what is expected from the Compton-Getting effect for many hours after the onset. The direction of the first-order anisotropy was not along the projection of local magnetic fields onto the scan plane of the detector and it was not affected by the polarity of the field either, indicating that the particles did not arrive at Ulysses through propagation along magnetic field lines and rather much of the anisotropy was produced by cross-field diffusion in the presence of a cross-field density gradient pointing toward the low latitude direction. All these observations are consistent with easy particles transport across the mean heliospheric magnetic fields. The apparent difficulty for the theory is that the observations require a cross-field diffusion that is too fast to be explained by random walk of field lines due only to supergranulation.
ASJC Scopus subject areas
- Space and Planetary Science