  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, that 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 Alfven 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 Alfven 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.
