Effect of magnetic discontinuities on the propagation of energetic particles: Ulysses observations of the onset of the March 1991 solar particle event - Sanderson, TR; Erdös, G; Balogh, A; Forsyth, RJ; Marsden, RG; Gosling, JT; Phillips, JL; Tranquille, C (JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, 2000)