Isotopes in the solar wind: New results from ACE, SOHO, and WIND

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Measuring the isotopic composition of the solar wind is interesting because it provides unique information on the isotopic composition of the solar atmosphere, the outer convective zone of the Sun, and the bulk Sun. Comparing the solar isotopic composition with other solar system samples can give clues about the early history of the solar system. If compared with the present-day interstellar medium, e.g., derived from interstellar pick-up ions in the interplanetary medium or from the anomalous cosmic-ray component, the solar isotopic composition yields valuable information on the galactic chemical evolution during the last 4.6 Gy and on the radial migration of the Sun within the galaxy. Solar isotopic abundances of volatile elements (He, Ne) have been used to put constraints on the internal transport of matter during the entire history of the Main-Sequence Sun and on the evolution of planetary atmospheres. More recently, the isotopic composition of refractory elements in different solar wind regimes has been used to infer the importance of fractionation processes operating between the radiative solar core, the outer convective zone, and the solar atmosphere, as well as between the solar atmosphere and the interplanetary plasma. The analysis of the isotopes of Mg, Si, and Ca as observed with the WIND/SMS, the SOHO/CELIAS, and the ACE/SWIMS isotope spectrometers indicate that the variability of isotopic abundance ratios in different solar wind regimes amounts to less than a few percent per mass unit, and that the overall isotopic composition of refractories in the solar wind is within the uncertainties identical to the terrestrial, lunar, and meteoritic composition. From observational evidence and from theoretical models on minor ion heating and acceleration in the corona it seems clear that coronal hole high speed streams provide the most authentic samples of the isotopic composition of the solar photosphere.