Isotopic Fractionation in 3He-rich SEP Events

A well established characteristic of He-3-rich solar energetic particle events is a pattern of abundance enhancements relative to normal solar system values that increase approximately monotonically with atomic number (or mass), leading to enhancements that can exceed 100x for the heaviest elements relative to oxygen. In a 2018 paper, Mason and Klecker (M&K) suggested that the heavy element enhancements could be due to characteristics of the stopping powers of low-energy nuclei penetrating thin layers of H gas or plasma. We present results of a new calculation of enhancements in a H gas and show them to be in reasonable agreement with the M&K results. Studies of heavy-element isotopic composition in 3He-rich events using data from ACE/SIS have also shown that heavier isotopes of an element typically exhibit significant abundance enhancements relative to lighter isotopes of the same element. We applied the new calculation to investigate whether the observed isotopic fractionation can be explained using the M&K H-gas model with the same parameters that account for the heavy-element abundances and found that the measured isotopic enhancements are significantly greater than the predicted values. A search of a larger parameter space was also not able to reconcile the elemental and isotopic enhancement values. However, this search did reveal trends in the dependence on the model parameters that merit further investigation.