The Solar Wind Ion Mass Spectrometer (SWIMS) on ACE has successfully measured the isotopic composition of sulfur in the solar wind. The figure above shows a histogram of the measured mass (m) of solar wind ions with 30 <= m <= 35. Included are the original data, "filtered" data points that have been smoothed across several adjacent mass channels, and a fit based on a model of the SWIMS response. Preliminary analysis yields ³⁴S/³²S ~= (4.3 +/- 0.6)% which is in good agreement with the meteoritic value of ³⁴S/³²S = 4.43%.

In contrast to the elemental abundance of sulfur, which can be accurately measured spectroscopically, the isotopic composition of sulfur on the Sun cannot be directly measured. Solar wind or solar particle measurements are thus the only means of determining the solar isotopic abundances of sulfur and most other elements.

For refractory elements the meteoritic isotopic composition is thought to represent the solar composition very well. This has been verified with measurements of Mg and Si in the solar wind.

Sulfur is a moderately volatile element, in contrast to the refractory elements Mg and Si. Thus the determination of its isotopic composition on the Sun can yield valuable information on possible fractionation mechanisms active during the formation of the early solar system.

Contributed by Robert F. Wimmer-Schweingruber of University of Bern, Switzerland.

See The The SWICS/SWIMS Home Page for more information about the SWIMS instrument.

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Last modified 12 February, 1999, Andrew Davis
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