Since its August 1997 launch, the Ultra-Low-Energy-Isotope spectrometer (ULEIS) onboard the ACE spacecraft has observed over two dozen solar energetic particle (SEP) events sufficiently large for isotopic measurements of neon at 1 MeV/nucleon. These events show remarkable variability of both the elemental and isotopic compositions (see also ACE News #22 & #30). The figure above shows the ACE/ULEIS 22Ne/20Ne versus Fe/Mg in the 0.83.0 MeV/nucleon energy range. The left panel is the 3He-"rich" events, events with 3He/4He > 0.007, and the right panel the 3He-"poor" events -- a division that attempts to distinguish between impulsive and gradual type SEP events, respectively. We note that even the 3He-"poor" events have, on average, 3He/4He ratios substantially (factor of 5) higher than the solar wind value of ~4.910-4. The dotted lines in the figure show the Fe/Mg ratio in the photosphere (Fe/Mg = 0.79) and the solar-wind neon isotope ratio (22Ne/20Ne = 0.073), while the solid lines show the best power-law fit to the data. The power-law indices are 1.05 +/- 0.25 (chi2/v = 0.3) for the 3He-"rich" events and 0.29 0.1 (chi2/v = 1.2) for the 3He-"poor" events. Breneman and Stone (1985) found that the enhancements and depletions of the elemental abundances of SEP events could be fit with a power-law in the charge to mass ratio, Q/M. If we assume that such a relationship also holds for the isotopes (see ACE news #30) then, assuming QMg/QFe = 0.66, which remains approximately true even as QFe and QMg vary with temperature, then 22Ne/20Ne should change as a power-law in Fe/Mg with a power-law index of 0.23 (dashed lines in the figure). This is in reasonable agreement with the 3He-"poor" events, but it is at least a factor of 4 too low for the 3He-"rich" data. If the energetic particles can be regarded as test particles in an electromagnetic field then their behavior should be completely governed by their velocity and Q/M. Because the difference in Q/M for 22Ne and 20Ne is only 10%, much smaller than the ~40% difference in Q/M for Mg and Fe, it is quite surprising that, for the 3He-"rich" data, the variations in 22Ne/20Ne are so large when compared to the variations in Fe/Mg.
Contributed by J. R. Dwyer and G. M. Mason of the University of Maryland.
See The ULEIS Home Page for more information on ACE ULEIS.
Last modified July 30 1999,
Andrew Davis