Hydrogen wall

Charge exchange processes in the outer heliosphere produce a population of hot hydrogen gas within the heliosphere, creating a “hydrogen wall” between the heliopause and bow shock . The heliospheric hydrogen wall scatters Lyα photons passing through it, producing a detectable absorption signature in observations of H I Lyα emission from nearby stars. This heliospheric absorption has been observed using observations from various space born laboratories and telescopes, and these observations have also yielded detections of analogous “astrospheric ” absorption from material surrounding the observed stars. The astrospheric detections dramatize the importance of understanding the heliospheric interaction, since similar interactions exist around other stars . .

Around each star  exists a region of hot hydrogen known as the hydrogen wall between the bow shock and the heliopause . The wall is composed of interstellar material interacting with the edge of the heliosphere

An alternative definition is that the heliopause is the magnetopause between the Solar System's magnetosphere and the galaxy's plasma currents .

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