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ACE News #81 - May 24, 2004
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Solar energetic particle nuclei much heavier than Fe and Ni [UH (ultraheavy) nuclei with Z>=30] were first detected in the 1960s using the Apollo 16 command module window as a track detector; recently, observations on the Wind spacecraft showed that UH nuclei are associated with 3He-rich flares. The UH nuclei in these studies were found to be much more abundant than in solar system material.
A systematic survey of UH abundances with mass numbers 78-220 amu was carried out using ULEIS from Sept. 1997 through April 2003 by summing together a large number of periods when 3He-rich flares were underway; the total collection time was 295.7 days. The left panel above shows the mass histogram of the Fe peak and above for the energy range 150-500 keV/nucleon. Although individual mass peaks are not resolved in this region, the broad peaks for the ranges 78-100 amu, 125-150 amu, and 180-220 amu are similar to features seen in solar system abundances. For this same long collection interval, the center panel shows differential fluence spectra for several representative species along with the UH mass groups 78-100, 125-150, and 180-220 amu. The 3He/4He ratio for this long period is about 10%, highly enriched compared to the solar wind. The spectra for the UH ions are similar to those for lighter species. The right panel shows the abundance enhancement of major species including UH nuclei plotted vs. the charge-to-mass ratio, Q/M, for charge state values typical of SEPs or, in the case of UH nuclei, of a 3.2 MK plasma. Notice that the enhancements are generally ordered by a power law in Q/M, as found previously for large shock-associated SEP events. Compared to solar system material the average enrichments are ~7 for Fe, ~40 for 78-100 amu, ~120 for 125-150 amu, and ~215 for 180-220 amu. In individual 3He rich events, the UH enhancements are at least a factor of 5 greater, and they often exceed the enhancement in the 3He/4He ratio.
These observations place new constraints on models for enrichment mechanisms in 3He-rich flares. Although a plasma resonance mechanism may still be required to enrich 3He, it seems difficult to see how this could simultaneously enrich the UH nuclei through higher harmonic resonances, since the UH nuclei as a group have an almost continuous range of Q/M ratios, and they also show extremely large enhancements, not the more modest factor of 5-7 for Ne-Fe previously addressed in such models. We note that the ordering of the enhancement by a power law in Q/M, with its well known association with shock-associated gradual SEP events, may be evidence that a "smoothed" shock is operating to enrich UH nuclei in these events.
Submitted by G. M. Mason, J. E. Mazur, J. R. Dwyer, J. R. Jokipii, R. E. Gold, and S. M. Krimigis; to appear in the May 1, 2004 issue of Astrophysical Journal. Comments and questions should be addressed to firstname.lastname@example.org
Last modified 24 May 2004, by