Swan observations of the solar wind latitude distribution and its evolution since launch

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.