SWAN is the only SOHO instrument that avoids looking towards the Sun. Instead, it maps most of the sky by ultraviolet light.
A general glow in this SWAN image comes from hydrogen atoms in a breeze of gas from interstellar space. The glow is brighter on the upwind (right) side. Ultraviolet rays from the Sun illuminate the atoms, but solar-wind protons break them up, so that they no longer glow. That is why the general glow is weaker on the downwind (left) side.
