Acceleration in a current sheet and heavy ion abundances in impulsive solar flares

The influence of collisional energy losses on stochastic particle acceleration in impulsive solar flares is considered in the context of preferential acceleration of heavy ions. It is shown that ion pre-acceleration in a reconnecting current sheet mitigates the effect of collisional energy losses, thus removing a strong sensitivity of the resulting anomalous abundances on the initial ion charge states. As an example, the expected Fe/O enhancement factors are computed and shown to be comparable with those observed with ACE SEPICA in a series of impulsive flares in 1998. One consequence of the model is that the preferential acceleration of heavy ions can occur only when the plasma gas pressure is large enough, beta approximate to m(e)/m(p), which may explain the observed correlation between the heavy ion enrichment and selective He-3 acceleration in impulsive flares.