Determination of the argon isotopic ratio of the solar wind using SOHO/CELIAS/MTOF

Authors: J. M. Weygand, F. M. Ipavich, P. Wurz, J. A. Paquette, and P. Bochsler
Reference: 1999 8th SOHO Workshop

Abstract: This study is about the first direct measurements of the 36Ar/38Ar ratio in the solar wind with the MTOF sensor of CELIAS on the SOHO spacecraft.

Argon is highly volatile and a minor element in the solar wind. Because of its volatility, inferences about the solar argon isotopic composition from planetary samples are problematic. However, it is possible to determine the solar isotopic composition quite reliably from solar wind observations. Such determinations have been made with the Apollo foil experiment predominantly for periods of slow solar wind and a value consistent with the terrestrial atmospheric 36Ar/38Ar ratio of 5.32 had been found.

CELIAS/MTOF has already been successfully used to examine the isotopic ratios of elements such as magnesium, neon, calcium, silicon, as well as nitrogen. Lunar soil derived 36Ar/38Ar ratios lie somewhatabove the terrestrial value, however, they represent flux averages which are integrated over long time periods and could be affected by fractionation effects during implantation and storage. Direct measurements provide the possibility to set limits to the variability of isotopic abundance ratios with different solar wind regimes, and hence, to obtain a clue on the importance of fractionation effects occurring in the solar wind.

From this study a preliminary 36Ar/38Ar abundance ratio of 5.8 +/- 1.1 which is consistent with indirect observations. This value is derived from approximately one day of slow solar wind. Furthermore, 20Ne/22Ne ratios as well as 20Ne/38Ar ratios are monitored and are found to be similar to published results. The result will be discussed in the context of solar wind fractionation models and of experimental evidence derived from in situ observations on refractory elements.

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Last Update: April 2, 1999, James Weygand