ENERGETIC-PARTICLE ABUNDANCES IN IMPULSIVE SOLAR-FLARE EVENTS

We report on the abundances of energetic particles from impulsive solar flares, including those from a survey of 228 He-3-rich events, with He-3/He-4>0.1, observed by the ISEE 3 spacecraft from 1978 August through 1991 April. The rate of occurrence of these events corresponds to similar to 1000 events yr(-1) on the solar disk at solar maximum. Thus the resonant plasma processes that enhance He-3 and heavy elements are a common occurrence in impulsive solar flares. To supply the observed fluence of He-3 in large events, the acceleration must be highly efficient and the source region must be relatively deep in the atmosphere at a density of more than 10(10) atoms cm(-3). He-3/He-4 may decrease in very large impulsive events because of depletion of He-3 in the source region. The event-to-event variations in He-3/He-4, H/He-4, e/p, and Fe/C are uncorrelated in our event sample. Abundances of the elements show a pattern in which, relative to coronal composition, He-4,C,N, and O have normal abundance ratios, while Ne, Mg, and Si are enhanced by a factor similar to 2.5 and Fe by a factor similar to 7. This pattern suggests that elements are accelerated from a region of the corona with an electron temperature of similar to 3-5 MK, where elements in the first group are fully ionized (Q/A = 0.5), those in the second group have two orbital electrons (Q/A similar to 0.43), and Fe has Q/A similar to 0.28. Ions with the same gyrofrequency absorb waves of that frequency and are similarly accelerated and enhanced. Further stripping may occur after acceleration as the ions begin to interact with the streaming electrons that generated the plasma waves.