ACE News #35
July 30, 1999

Variability of 22Ne/20Ne at 1 MeV/nucleon Measured by ACE/ULEIS

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.

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Last modified July 30 1999, Andrew Davis