SWAN Lyman-alpha whole sky map in ecliptic coordinates. Two
areas were not covered for safety reasons, around the Sun and
around the anti-solar direction. The color is coding the
intensity, in counts per second per pixel (one square degree),
which corresponds to 1.3 Rayleigh. A number of UV hot stars can
be identified, tracing the galactic plane. The rest of the
ubiquitous emission is due to solar UV Lyman alpha photons,
backscattered by Hydrogen atoms in the solar system. These H
atoms are coming from interstellar space, and are approaching
the Sun down to about 2 AU, in the direction of the incoming
flow (ecliptic coordinates, longitude 254 deg, latitude 7 deg).
A maximum of Lyman-alpha intensity surrounds this upwind
direction. In the opposite direction, the emission is weaker by
a factor of 3.5, because most atoms have been destroyed by
charge-exchange with solar wind protons, creating a cavity void
of Hydrogen atoms in the downwind direction. A detailed
comparison of such Lyman-alpha maps will allow to d