Determination of the 36Ar/38Ar isotopic abundance ratio of the solar wind using SOHO/CELIAS/MTOF

Abstract

This study is on the first direct measurements of the 36Ar/38Ar isotopic ratio in both the interstream (IS)-associated and coronal hole (CH)-associated solar wind. Two summed mass spectra are compiled for each solar wind region using the Mass Time-Of-Flight (MTOF) sensor of the Charge, Element, and Isotope Analysis System (CELIAS) instrument on the Solar and Heliospheric Observatory (SOHO) spacecraft. A detailed analysis of over 9.2 d of nonconsecutive IS-associated solar wind speed (395 ± 25 km/s) places the 36Ar/38Ar ratio at 5.6 ± 0.7. A similar analysis for a CH-associated solar wind speed (525 ± 25 km/s), with ∼22.8 nonconsecutive days of CH-associated solar wind speed data, derived a similar 36Ar/38Ar ratio of 5.5 ± 0.6. The error associated with these measurements is essentially the instrumental uncertainty. Both the solar wind 36Ar/38Ar isotopic ratios found in this study are consistent with the terrestrial 36Ar/38Ar ratio as well as previous reported ratios for solar wind examined in this study. A comparison of these results suggests little or no isotopic fractionation occurring between the two solar wind regimes. This study presents the results in the context of solar wind fractionation models and in relation to experimental evidence derived from in situ observations on refractory elements.