Backscattered Solar Lyman-$\alpha $ Emission as a Tool for the Heliospheric Boundary Exploration

This review summarizes our current understanding of the outer heliosphere and local interstellar medium (LISM) inferred from observations and modeling of interplanetary Lyman- α emission. The emission is produced by solar Lyman- α photons (121.567 nm) backscattered by interstellar H atoms inflowing to the heliosphere from the LISM. Studies of Lyman- α radiation determined the parameters of interstellar hydrogen within a few astronomical units from the Sun. The interstellar hydrogen atoms appeared to be decelerated, heated, and shifted compared to the helium atoms. The detected deceleration and heating proved the existence of secondary hydrogen atoms created near the heliopause. This finding supports the discovery of a Hydrogen Wall beyond the heliosphere consisting of heated hydrogen observed in HST/GHRS Lyman- α absorption spectra toward nearby stars. The shift of the interstellar hydrogen bulk velocity was the first observational evidence of the global heliosphere asymme-I. try confirmed later by Voyager in situ measurements. SOHO/SWAN all-sky maps of the backscattered Lyman-α intensity identified variations of the solar wind mass flux with heliolatitude and time. In particular, two maxima at mid-latitudes were discovered during solar activity maximum, which Ulysses missed due to its spe-cific trajectory. Finally, Voyager/UVS and New Hori-zons/Alice UV spectrographs discovered extraheliospheric Lyman- α emission. We review these scientific break-throughs, outline open science questions, and discuss potential future heliospheric Lyman- α experiments.