Composition of the local interstellar medium as diagnosed with pickup ions

Abstract

Pickup ions provide us with a powerful tool to probe remote regions in and beyond the heliosphere and determine the composition of the neutral gas in these regions. Interstellar pickup ions, observed inside the heliosphere at 3–5 AU from the Sun, are created by charge exchange and photoionization of the local interstellar gas that penetrates deep into the heliosphere. Comprehensive measurements of H, He, C, N, O, and Ne, especially with the Solar Wind Ion Composition Spectrometer (SWICS) on both the Ulysses and ACE spacecraft, are giving us a wealth of data that we use to infer the chemical and physical properties of the Local Interstellar Cloud (LIC). Using updated and improved measurements of pickup ions and state-of-the-art modeling, we find that in the LIC the proton and neutral hydrogen densities are respectively (0.061 ± 0.018) and (0.176 ± 0.019) cm −3. The neutral helium density is (0.0154 ± 0.0015) cm −3 and the ionized helium density (0.0083 ± 0.0021) cm −3, giving a LIC total gas density of hydrogen plus helium of (0.261 ± 0.026) cm −3. Using these updated densities, the location of the heliospheric termination shock is calculated to be at about 96 AU in 2002, and the crossing of Voyager 1 through this shock is predicted to occur between 2003 and 2009. The metallicity of the gas in the LIC is found to be comparable to that of the Protosolar Cloud. This unexpected result may be explained by a recent model of quasi-continuous excess infall of extra-galactic material that is only moderately processed by stellar nucleosynthesis [Astrophys. J. 578 (2002) 862].