He⁺ Ions Comoving with the Solar Wind in the Outer Heliosphere

The solar wind emitted from the solar corona is a highly ionized plasma composed of electrons, protons, He²⁺ ions, and a small fraction of heavier ions. Additionally, interstellar neutral (ISN) atoms ionized in the heliosphere form populations of pickup ions (PUIs). The two most abundant PUIs are of H⁺ and He⁺. Some solar wind ions are neutralized due to charge exchange with ISN atoms. Besides the neutral solar wind created by this process, a population of He⁺ ions is formed from solar wind He²⁺ ions, with kinetic properties inherited from solar wind ions. The rate of production of these He⁺ ions increases significantly with solar wind speed. Consequently, in the fast solar wind, over 10% of solar wind He²⁺ turns into He⁺ along the path to the termination shock. There have been very few observations of this population so far. Here we show that observations from the Solar Wind Around Pluto (SWAP) instrument on New Horizons contain a signature of this population. Namely, solar wind He⁺ ions cause an enhanced flux just below the cutoff energy of H⁺ PUIs. We identify 17 daily spectra collected by SWAP with this feature during steady solar wind conditions. We find that the fit parameters of the H⁺ PUI model are considerably affected when the He⁺ ion population is significant. Moreover, we note that some of the observations are likely also affected by other heavy solar wind ions, e.g., oxygen and carbon.