ENERGETIC PARTICLES FROM IMPULSIVE SOLAR-FLARES

Recent observations of the solar energetic particles (SEPs) from impulsive flares are reviewed. In contrast to the particles seen in large proton events, those associated with impulsive flares have an unusual composition that is now understood to be dominated by the effects of wave-particle interactions near the flare site. Impulsive flare events are characterized by high fluxes of electrons that are observed directly and through the type III radio emission that they produce as they propagate outward from the Sun. Ion observations in these events show dramatic enhancements in abundance ratios such as 3He/4He and Fe/O. The heavier ions are observed to be highly ionized by electron collisions in the flare-heated plasma. It is proposed that the observed particles are accelerated stochastically on open field lines near the flare loop by plasma waves generated within the flare loop, and an experimental test of this proposal is suggested. Effects of these low-frequency plasma processes are not observed in the radio, optical, X-ray, or γ-ray emission from the flare but can only be seen in the particle measurements. Comparison of the SEP abundances with those derived from γ-ray lines, emitted from the ambient flare plasma, can distinguish enhancements caused during acceleration from those that occurred during transport of material up into the corona. Study of the plasma processes that control element abundances at the Sun may have wider application to the understanding of abundances observed in other regions of the Galaxy where direct particle observations are not possible.