Perpendicular transport of solar energetic particles in heliospheric magnetic fields

This paper presents direct observational evidence for large perpendicular diffusive flows of solar energetic particles in heliospheric magnetic fields. Using anisotropy measurements of 40-92 MeV protons obtained by the High-Energy Telescope of the COSPIN consortium on Ulysses at high heliographic latitudes, we find that during several gradual solar particle events, for periods of many hours, the observed particle flow is directed at significant angles to the measured local magnetic field direction, which can only be explained by cross-field diffusion. The variation of the particle flow direction with the magnetic field direction seen in the 2000 July 14 solar energetic particle event has allowed us to derive a large κ_⊥/κ _∥ ratio of 0.25 with an error bar of less than 0.05. An upper limit κ_A/κ_⊥ ≪ 0.17 is also found by analyzing the dependence of the particle anisotropy on the magnetic field polarity. The observation suggests that perpendicular diffusion of solar energetic particles cannot be neglected even in the early phase of a solar particle event.