Z-rich solar particle event characteristics 1972--1976

A systematic analysis of the hourly average Z> or =3 rate date (1972) September--1976 December) measured with the JHU/APL IMP 7 and IMP 8 detectors has revealed 13 short-lived events that have greatly enhanced Z> or = 3 fluxes. In addition to confirming the results of earlier studies that all 3He-rich events are Fe-rich but not vice versa, we find that these 13 enriched events have several remarkable characteristics: (1) they are generally not associated with major solar flares but appear to be associated with low levels of activity (subflares) in western-hemisphere solar active regions that are located very close to the spacecraft's high coronal interplanetary magnetic field connection longitude; (2) they have very large and prolonged outward streaming anisotropies, sometimes persisting approx.1 day; (3) the spectra indices measured for p, α, and Z> or =3 particles during the times of maximum flux for the Z-rioch events are identical within errors to those measured in large flare events, while small impulsive and corotating events generally show a softer spectrum; and (4) Z-rich events appear to be associated with the low-speed wind that precedes the onset of solar wind streams. We conclude from this study that the same type of acceleration process that is responsible for the large proton events is responsible for the small Z-rich events, and therefore that the Z+ or =3 and 3He enrichments are more likely due to enhanced abundances in the preaccelerated plasma than to preferential enrichment during the acceleration process. This interpretation is complementary to recent findings concerning abundances and coronal emission structure of low-speed solar wind. We conclude that compositional studies of low-energy solar particle events offer a good diagnostic of the flare acceleration process and possible inhomogeneities in the solar atmosphere, but are poor indicators of solar composition on a global scale