Updated Measurements of the Isotopic Composition of Solar Energetic Particles

Recent results from the Advanced Composition Explorer (ACE) have shown that solar energetic particle (SEP) isotopic composition in large gradual events can vary considerably (by factors of >3) from event to event, apparently due to mass fractionation processes during particle acceleration and/or transport. To first order, this mass fractionation seems to scale as a power law in the ionic charge to mass ratio, Q/M, similar to what is observed for SEP elemental fractionation. However, differences from power-law scaling are required for most events to completely account for the observed abundances. Characterizing the actual fractionation patterns is necessary in order to use these data to better constrain the coronal source abundances and to understand the fractionation processes. In the >6 years since the launch of ACE, we have obtained SEP isotope measurements of abundant elements from C to Ni in >40 large SEP events. We find that the isotopic and elemental abundance enhancements are strongly correlated. By assuming isothermal conditions and using calculated equilibrium charge states, we examine deviations in the fractionation patterns from simple power-law scaling. This work was supported by NASA at Caltech (under grants NAG5-6912 and NAG5-12929), JPL, and GSFC.