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ACE News #48 - August 25, 2000

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Oxygen Isotopes in the Fast Solar Wind: ACE/SWIMS

Oxygen is the most abundant element in the solar system (and the universe) after H and He, and the lightest element with three stable isotopes. Oxygen is a volatile element; it did not condense completely in meteorites during their formation in the early solar system. The Sun is the only source from which one can accurately determine the oxygen isotopic composition of the nebula from which the solar system formed.

Oxygen occurs in highly volatile phases of the solar system, such as water, as well as in refractory phases, for example in silicates. Because of this, the differences between the isotopic composition of oxygen in various bodies in the solar system and that of the Sun are a sensitive measure of the gas to dust ratio in the meteorite-forming region in the early solar system. They can also yield valuable information on the temperature history in that region.

Known fractionation processes between the photosphere and the solar wind do not alter the isotopic composition by more than a few percent per mass unit. They seem to affect the slow solar wind most, and hence it is expected that high-speed streams carry the least fractionated sample of the isotopic composition of the Sun.

With the Solar Wind Isotope Mass Spectrometer (SWIMS) on ACE it has, for the first time, been possible to measure the oxygen isotopic composition in the fast solar wind. In the figure, we show one years worth of data accumulated in the high-speed solar wind (exceeding 500 km/s). Because of their low abundances, the heavy isotopes of oxygen are not easily measured. We obtain an isotopic abundance ratio 16O/18O of 446 +/- 90 which is consistent with, but below the terrestrial value of 498. It is also in excellent agreement with a previous measurement in the slow solar wind (slower than 400 km/s) by the MASS instrument on Wind.

The quoted uncertainty is the sum of the squares of the statistical uncertainty in the 18O counts, the background, and estimates for systematic and instrumental uncertainties. Background due to solar wind protons has been removed using the mass-per-charge resolving capabilities of SWIMS. The remaining background comes from solar wind helium particles and is inherent in this type of instruments. It has been accounted for in the instrument model function which is used for data analysis. The 18O counts show the expected mass-per-charge distribution when compared with that of 16O.

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

See The SWICS/SWIMS Home Page for additional information on ACE/SWIMS.

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Last modified 25 August 2000, by Andrew Davis