ENERGETIC-PARTICLE ABUNDANCES IN IMPULSIVE SOLAR-FLARE EVENTS

We report on the abundances of energetic particles from impulsive sola r 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 thro ugh 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 ar e a common occurrence in impulsive solar flares. To supply the observe d fluence of He-3 in large events, the acceleration must be highly eff icient 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 so urce 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 eleme nts 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 enhance d by a factor similar to 2.5 and Fe by a factor similar to 7. This pat tern suggests that elements are accelerated from a region of the coron a 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 gr oup have two orbital electrons (Q/A similar to 0.43), and Fe has Q/A s imilar to 0.28. Ions with the same gyrofrequency absorb waves of that frequency and are similarly accelerated and enhanced. Further strippin g may occur after acceleration as the ions begin to interact with the streaming electrons that generated the plasma waves.