An Update on Ultra-Heavy Elements in Solar Energetic Particles above 10 MeV/Nucleon

Abstract

Measurements below several MeV/nucleon from Wind/LEMT and ACE/ULEIS show that elements heavier than Zn (Z=30) can be enhanced by factors of ~100 to 1000, depending on species, in ^3He-rich solar energetic particle (SEP) events. Using the Solar Isotope Spectrometer (SIS) on ACE we find that even large SEP (LSEP) shock-accelerated events at energies from ~10 to >100 MeV/nucleon are often very iron rich and might contain admixtures of flare seed material. Studies of ultra-heavy (UH) SEPs (with Z>30) above 10 MeV/nucleon can be used to test models of acceleration and abundance enhancements in both LSEP and ^3He-rich events. We find that the long-term average composition for elements from Z=30 to 40 is similar to standard solar system values, but there is considerable event-to-event variability. Although most of the UH fluence arrives during LSEP events, UH abundances are relatively more enhanced in ^3He-rich events, with the (34<Z<40)/O ratio on average more than 50 times higher in ^3He-rich events than in LSEP events. At energies >10 MeV/nucleon, the most extreme event in terms of UH composition detected so far took place on 23 July 2004 and had a (34<Z<40)/O enhancement of ~250–300 times the standard solar value.