We report on the abundances of energetic particles from impulsive solar flares, including those from a survey of 228 3 He-rich events, with 3He/4He > 0.1, observed by the ISEE 3 spacecraft from 1978 August through 1991 April. The rate of occurrence of these events corresponds to ~1000 events yr−1 on the solar disk at solar maximum. Thus the resonant plasma processes that enhance 3He and heavy elements are a common occurrence in impulsive solar flares. To supply the observed fluence of 3He 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 1010 atoms cm−3 .3He/4He may decrease in very large impulsive events because of depletion of 3He in the source region.
The event-to-event variations in 3He/4He, H/4He, e/p, and Fe/C are uncorrelated in our event sample. Abundances of the elements show a pattern in which, relative to coronal composition, 4He, C, N, and O have normal abundance ratios, while Ne, Mg, and Si are enhanced by a factor ~2.5 and Fe by a factor ~7. This pattern suggests that elements are accelerated from a region of the corona with an electron temperature of ~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 ~ 0.43), and Fe has Q/A ~ 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.
Subject headings: Sun: abundances — Sun: flares — Sun: particle emission