Element Abundances in Impulsive Solar Energetic Particle Events

Abstract

Abstract We outline and discuss a model for the enhanced abundances of trans-Fe elements in impulsive solar energetic particle (SEP) events, where large mass-dependent abundance enhancements are frequently seen. It comes about as a variation of the ponderomotive force model for the first ionization potential (FIP) effect, i.e., the increase in coronal abundance of elements like Fe, Mg, and Si that are ionized in the solar chromosphere relative to those that are neutral. In this way, the fractionation region is placed in the chromosphere and is connected to the solar envelope, allowing the huge abundance variations to occur, which might otherwise be problematic with a coronal fractionation site. The principal mechanism behind the mass-independent FIP fractionation becoming the mass-dependent impulsive SEP fractionation is the suppression of acoustic waves in the chromosphere. The ponderomotive force causing the fractionation must be due to torsional Alfvén waves, which couple much less effectively to slow modes than do shear waves, and upward-propagating acoustic waves deriving from photospheric convection must be effectively mode-converted to fast modes at the chromospheric layer, where Alfvén and sound speeds are equal, and subsequently totally internally reflected. We further discuss observations of the environments thought to be the source of impulsive SEPs and the extent to which the real Sun might meet these conditions.