A survey of approximately 1 MeV/nucleon solar flare particle abundances, in the Z ? 1-26 range, during the 1973-1977 solar minimum period

We have surveyed the abundances of the major elements over the range H--Fe in solar flare energetic particles near 1 MeV nucleon/sup -1/ for a large number of flares during the period 1973--1977. The observations were carried out in interplanetary space using the University of Maryland/Max-Planck-Institut instrumentation on the IMP 8 spacecraft. The survey considered two types of solar flare events: (1) large events from which the average abundances were deduced, and (2) events which had significant abundance differences from the average. The average abundances near 1 MeV nucleon/sup -1/ for the large events are similar to previous abundance compilations for energies > or approx. =10 MeV nucleon/sup -1/ based on limited number of flare events. Where comparisons can be made, the abundances are similar to solar photospheric and coronal values, the largest differences being Ne and Mg, which are relatively more abundant in the energetic particles. For the large events, variations in the abundances from flare to flare, and with time during single flares, give no indication that the physical mechanisms preferentially accelerate any particular species over the energy range of the observations. Therefore, it is suggested that the reported photospheric-coronal abundances of Ne and Mg might be toomore » low. The events with the most extreme abundance differences from the average were all small solar flare events, consistent with a pattern for the entire survey wherein abundance variations from flare to flare were found to increase with decreasing peak intensity of the event. The types of abundance anomalies found included simultaneous /sup 3/He and heavy nucleus enrichments for O--Fe, carbon depletion reported previously, and two additional types: (a) an event with /sup 3/He enrichment and no heavy ion enhancements, and (b) a /sup 3/He-rich event with only Si--Fe enriched. Implications for a plasma-wave heating model to produce /sup 3/He and Fe enrichment are discussed.« less