Effect of magnetic discontinuities on the propagation of energetic particles: Ulysses observations of the onset of the March 1991 solar particle event

We present observations of energetic ions from the Cosmic Ray and Solar par ticle Investigation (COSPIN) and Heliosphere Instrument for Spectra Composi tion and Anisotropy at Law Energy (HISCALE) instruments on board the Ulysse s spacecraft. We examine ill detail the onset of energetic ions observed du ring the March 1991 series of solar events. Active region 6555, at E28 at 2 247 UT on March 22, 1991 (day 81), was most likely responsible for the high -energy protons subsequently seen at Ulysses. A slow increase in the high-e nergy (similar to 50 MeV) proton intensity, made up of many small but rapid intensity increases, began around 9 hours after the onset of the event, ta king 1.5 days to reach maximum intensity. On the flank, and in front of a c oronal mass ejection (CME), coming From a previous event, we observed a reg ion with a drop in the magnetic field strength. This region was populated w ith hot shocked plasma and, owing to the low magnetic field pressure, had a n ion plasma beta >1. Mirror mode waves, with discontinuities appearing at the edges of the waves, produced a larger-than-average number of magnetic f ield discontinuities, a large fraction of the discontinuities being aligned with their normals oriented along the radial direction, suggesting that th ey were being convected outward by the CME. These discontinuities acted as a series of barriers to the propagation of the energetic solar particles co ming from the solar event, delaying substantially the onset of the particle event at Ulysses.