Solar particle abundances at energies of greater than 1 MeV per nucleon and the role of interplanetary shocks

Abstract

We have examined the abundances of elements in large solar energetic particle events in the energy range 2-12 MeV per nucleon. There are large variations in abundances from event to event but the data show an organization. As found by previous workers, we find that the abundances relative to mean values vary approximately monotonically as a function of mass (A), except for 4He. Some events show a gradual depletion of heavy ions, whereas a smaller number show a gradual increase. In this paper we show for the first time a further organization of abundance data which depends on the longitude of the source region. The abundances as a function of longitude can be understood in terms of two particle sources, namely (1) particles originating out of flare-heated material and (2) particles accelerated at coronal and interplanetary shocks. The greatest depletions of heavy elements occur when source regions are near central meridian which is also where the strongest shocks originate. The depletions are matched by a steepening of the spectrum and can be understood in terms of diffusive shock acceleration. The greatest enhancements of heavy elements occur when source regions are near west 60○. In this situation the observer can be magnetically connected to flaring regions and have access to flare-heated material which is enriched in heavy elements.