SWAN is the first space instrument dedicated to the monitoring of the latit
ude distribution of the solar wind by the Lyman alpha method. The distribut
ion of interstellar H atoms in the solar system is determined by their dest
ruction during ionization charge-exchange with solar wind protons. Maps of
sky Ly-cr emission have been recorded regularly since launch. The upwind ma
ximum emission region deviates strongly from the pattern that would be expe
cted from a solar wind that is constant with latitude. It is divided in two
lobes by a depression aligned with the solar equatorial plane, called the
Lyman-alpha groove, due to enhanced ionization along the neutral sheet wher
e the slow and dense solar wind is concentrated. The groove (or the anisotr
opy) is more pronounced in 1997 than in 1996, but it then decreases between
1997 and 1998.