Variations of Heavy Ion Abundances Relative to Proton Abundances in Large Solar Energetic (E > 10 MeV) Particle Events.
2019 July 22
The elemental composition of heavy ions (with atomic number Z > 2) (hi-Z) in large gradual E > 10 MeV nuc1 SEP events has been extensively studied in the 2-15 MeV nuc1 range to determine the acceleration processes and transport properties of SEPs. These studies invariably are based on abundances relative to those of a single element such as C or O and often neglect H and He, the elements of primary interest for space weather. The total radiation of an SEP event is determined not only by the H and He properties but also by those of hi-Z ions whose abundances and variations relative to H from one event to another are unknown. We report a study to determine those variations in a group of 15 large SEP events over the period 2000 to 2015. Five hi-Z ions (He, C, O, Mg, & Fe) were selected to determine variations of their fluences relative to those of H in the 13.5-50.7 MeV nuc1 energy range for each SEP event. Our average hi-Z abundance ratios slightly exceed those reported by  at lower energies, with the Fe event abundances showing the largest standard deviation of an order of magnitude. The event abundances were weakly correlated with H fluences and strongly correlated with speeds Vcme of associated coronal mass ejections (CMEs). These correlations may be evidence of streaming limits in the shock regions of H in the largest events.