Composition of Interstellar Neutrals and the Origin of Anomalous Cosmic Rays

Abstract

Knowledge of the elemental composition of the interstellar gas is of fundamental importance for understanding galactic chemical evolution. In addition to spectroscopic determinations of certain element abundance ratios, measurements of the composition of interstellar pickup ions and Anomalous Cosmic Rays (ACRs) have provided the principal means to obtain this critical information. Recent advances in our understanding of particle acceleration processes in the heliosphere and measurements by the Voyagers of the energy spectra and composition of energetic particles in the heliosheath provide us with another means of determining the abundance of the neutral components of the local interstellar gas. Here we compare the composition at the termination shock of six elements obtained from measurements of (a) pickup ions at ∼5 AU, (b) ACRs in the heliosphere at ∼70 AU, and (c) energetic particles as well as (d) ACRs in the heliosheath at ∼100 AU. We find consistency among these four sets of derived neutral abundances. The average interstellar neutral densities at the termination shock for H, N, O, Ne and Ar are found to be 0.055±0.021 cm⁻³, (1.44±0.45)×10⁻⁵ cm⁻³, (6.46±1.89)×10⁻⁵ cm⁻³, (8.5±3.3)×10⁻⁶ cm⁻³, and (1.08±0.49)×10⁻⁷ cm⁻³, respectively, assuming the He density is 0.0148±0.002 cm⁻³.