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 modeling interplanetary Lyman-$\alpha$ emission. The emission is produced by solar Lyman-alpha photons (121.567 nm) backscattered H atoms inflowing to LISM. Studies radiation determined parameters hydrogen within a few astronomical units Sun. appeared be decelerated, heated, shifted compared helium atoms. detected deceleration heating proved existence secondary created near heliopause. finding supports discovery Hydrogen Wall beyond consisting heated observed in HST/GHRS absorption spectra toward nearby stars. shift bulk velocity was first observational evidence global asymmetry confirmed later Voyager situ measurements. SOHO/SWAN all-sky maps intensity identified variations wind mass flux with heliolatitude time. In particular, two maxima at mid-latitudes were discovered during activity maximum, which Ulysses missed due its specific trajectory. Finally, Voyager/UVS New Horizons/Alice UV spectrographs extra-heliospheric We these scientific breakthroughs, outline open science questions, discuss potential future heliospheric experiments.