A possible enrichment of heavy and ultraheavy ions in solar energetic particle events due to the effect of Coulomb losses

Abstract

In surveys of impulsive solar energetic particle events a systematic enrichment of heavy ion abundances compared to solar system abundances has been observed, with enhancement factors increasing from ∼ 7 for iron ions to ∼ 200 for ultra heavy (UH) ions in the mass range 180-220. Energy spectra and relative abundances of accelerated ions generally depend on the combined effects of mass per charge dependent acceleration, and charge stripping and coulomb losses, if the acceleration occurs in a sufficiently dense environment. We apply a model including stochastic acceleration, charge transfer processes, Coulomb losses and escape from the acceleration region to the acceleration of ions in the mass range of oxygen to lead to compute energy spectra and relative abundances in the energy range 0.1 to 0.3 MeV/nuc, assuming a range of values of the power-law index of magnetic turbulence (S). We find for S ≥ 2 preferential acceleration of ions starting already for Fe, systematically increasing with mass number. For S ∼ 2.5 the model predictions are consistent with the systematic increase of the observed average enhancement factors to ∼ 200 for ultraheavy ions.