2025AJ....169..275G

During 2023 September the Alice ultraviolet spectrograph on the New Horizons (NH) spacecraft was used to map diffuse Lyα emission over most of the sky, at a range of ∼56.9 au from the Sun. At that distance, models predict that the interplanetary medium Lyα emissions result from comparable amounts of resonant backscattering of the solar Lyα line by interstellar hydrogen atoms (H I) passing through the solar system, in addition to an approximately isotropic background of ∼50 ± 20 R from the local interstellar medium (LISM). The NH observations show no strong correlations with nearby cloud structures of the LISM or with expected structures of the heliosphere, such as a hydrogen wall associated with the heliopause. To explain the relatively bright and uniform Lyα of the LISM, we propose that hot, young stars within the Local Hot Bubble shine on its interior walls, photoionizing H I atoms there. Recombination of these ions can account for the observed ∼50 R Lyα background, after amplification of the diffuse Lyα by resonant scattering, although sophisticated (i.e., 3D) radiative transfer models should be used to confirm this conjecture. Future observations of the diffuse Lyα, with instruments capable of resolving the line profile, could provide a new window on H I populations in the LISM and heliosphere. The NH Alice all-sky Lyα observations presented here may be repeated at some point in the future, if resources allow, and the two maps could be combined to provide a significant increase in angular resolution.