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The mass spectrometric determinations of the isotopic composition of helium in the solar wind obtained from (1) the Apollo Solar Wind Composition (SWC) experiment, (2) the Ion Composition Instrument (ICI) on the International Sun Earth Explorer 3 (ISEE-3), and (3) the Solar Wind Composition Spectrometer (SWICS) on Ulysses are reviewed and discussed, including new data given by Gloeckler and Geiss (1998). Averages of the 3He/4He ratio in the slow wind and in fast streams are given. Taking account of separation and fractionation processes in the corona and chromosphere, 3He/4He = (3.8 ± 0.5) × 10−4 is derived as the best estimate for the present-day Outer Convective Zone (OCZ) of the sun. After corrections of this ratio for secular changes caused by diffusion, mixing and 3He production by incomplete H-burning (Vauclair, 1998), we obtain (D + 3He)/H = (3.6±0.5) × 10−5 for the Protosolar Cloud (PSC). Adopting 3He/H = (1.5±0.2) × 10−5 for the PSC, as is indicated from the 3He/4He ratio in the ‘planetary gas component’ of meteorites and in Jupiter (Mahaffy et al., 1998), we obtain (D/H)protosolar = (2.1 ± 0.5) × 10−5. Galactic evolution studies (Tosi, 1998) show that the measured D and 3He abundances in the Protosolar Cloud and the Local Interstellar Cloud (Linsky, 1998; Gloeckler and Geiss, 1998), lead to (D/H)primordial = (2 – 5) × 10−5. This range corresponds to a universal baryon/photon ratio of (6.0 ± 0.8) × 10−10, and to Ωb = 0.075 ± 0.015.

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