Population of He+ Ions Co-moving with the Solar Wind in the Outer Heliosphere

The solar wind emitted by the Sun is composed mostly of protons, a few percent He⁺⁺ ions, and a small fraction of highly ionized heavier species. Interstellar neutral atoms enter the heliosphere, where they can be ionized, largely due to charge-exchange with the solar wind plasma. As a result, a new population is incorporated into the solar wind called pickup ions. The most abundant pickup ions are H⁺ and He⁺. The distribution functions of the pickup ions show a cutoff at the double the solar wind speed in the Sun's reference frame. Recent observations by the Solar Wind Around Pluto (SWAP) instrument on the New Horizons spacecraft extend pickup ion observations into the distance solar wind, out to 40 au (McComas et al. 2017, ApJS 233:8). That study found a significant enhancement in a fraction of the SWAP spectra just below the H⁺ pickup ion energy cutoff. In this study, we provide calculations to show that this enhancement is likely a signature of He⁺ ions co-moving with the solar wind. The solar wind emitted from the solar corona usually does not contain a significant contribution of He⁺ ions. However, some of He⁺⁺ ions emitted from the solar corona are partially neutralized due to single charge-exchange collisions with the interstellar neutral atoms. The partially neutralized He⁺ ions created in this process retain properties of the parent He⁺⁺ ions. The charge-exchange cross section increases as the solar wind speed increases. Consequently, this population is most often observed for high He⁺⁺ abundances in the solar wind and high speed of the solar wind. We discuss consequences for the interpretation of outer heliospheric pickup ion spectra.