The determination of the solar wind argon abundance with SOHO/CELIAS/MTOF

Authors: J.M. Weygand, F.M. Ipavich, P. Wurz, J.A. Paquette, and P. Bochsler
Reference: Submitted Solar Wind 9 Paper


This study is about the first direct measurements of the solar wind argon elemental abundance with the Mass Time Of Flight (MTOF) sensor of the CELIAS instrument 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 abundance from planetary samples are impossible. However, it is possible to determine the solar argon abundance from solar wind observations. Spectrograms of the solar wind are obtained with the high-resolution mass spectrometer SOHO/CELIAS/MTOF. Obtaining the argon abundance from the solar wind is straightforward since both the 36Ar and 38Ar are not contaminated by doubly charged ions of higher mass, and neither lies siginificantly within the distribution of another mass peak. The main difficulty is the low signal to noise ratio in both argon peaks.

Relating argon (FIP ˜ 15.7 eV) to calcium (FIP ˜ 6.1 eV) makes it possible to investigate the FIP effect for another two elements with different FIP properties, but neighboring masses (like for Na/Ne). Using neighboring masses (i.e., masses with similar M/Q values) helps minimize the solar wind mass fractionation affects. The preliminary Ca/Ar abundance ratio for the slow solar wind appears to be about 1.6 ±(0.5) and for the fast solar wind about 1.4±0.4. The slow solar wind value appears to be less than the previously published SEP values, and the fast solar wind value is within of a factor of two of the photospheric Ar/Ca abundance ratio (0.91±0.01) [Grevesse and Sauval, 1998] which is consistent with a weak FIP effect in the coronal hole associated solar wind.

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Last Update: April 2, 1999, James Weygand