FIRST RESULTS FROM SWAN LYMAN-ALPHA SOLAR-WIND MAPPER ON SOHO

After one year of almost flawless operation on board the SOHO spacecra ft poised at L1 Lagrange point, we report the main features of SWAN ob servations. SWAN is mainly dedicated to the monitoring of the latitude distribution of the solar wind by the La method. Maps of sky LQ emiss ions were recorded throughout the year. The region of maximum emission , located in the upwind hemisphere,deviates strongly from the pattern that could be expected from a solar wind constant with latitude. It is divided into two lobes by a depression aligned with the solar equator ial plane called the Ly alpha groove already noted in 1976 Prognoz dat a. The north lobe is much brighter than the south lobe. These two char acteristics can be explained qualitatively by an enhanced ionization a long the neutral sheet where the slow solar wind is concentrated, whic h results from the higher low-latitude solar wind mass flux as measure d by Ulysses. The groove is the direct imprint on the sky of the enhan ced carving by the slow solar wind, at this time of solar minimum, whe n the tilt angle of the neutral sheet is small. The question is still pending to predict what will happen with the ascending phase of the so lar cycle. Observations of comets are briefly mentioned, with the abil ity of SWAN to monitor the H2O production of many comets. Operations o f the instrument are briefly described, including some instrumental pr oblems which could be solved by software modifications sent to the ins trument.