Whitepaper #275 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Main topics: space weather applications; basic research. Additional topics: outer heliosphere; inner heliosphere; other space weather; space-based missions/projects.
This supports development and deployment-to-space of a high-resolution spectrograph to distinguish different populations of H atoms that directly interact at the interface of the heliosheath, the region where the solar wind is subsonic. Resulting science would support Voyagers, IBEX, IMAP, New Horizons, and Interstellar Probe mission observations, and improve characterization of the solar wind.