SWAN - A STUDY OF SOLAR-WIND ANISOTROPIES ON SOHO WITH LYMAN-ALPHA SKY MAPPING

On board the SOHO spacecraft poised at L1 Lagrange point, the SWAN ins trument is mainly devoted to the measurement of large scale structures of the solar wind, and in particular the distribution with heliograph ic latitude of the solar wind mass flux. This is obtained from an inte nsity map of the sky Lyman cu emission, which reflects the shape of th e ionization cavity carved in the flow of interstellar K atoms by the solar wind. The methodology, inversion procedure and related complicat ions are described. The subject of latitude variation of the solar win d is shortly reviewed: earlier Lyman alpha results from Prognoz in 197 6 are confirmed by Ulysses. The importance of the actual value of the solar wind mass flux for the equation of dynamics in a polar coronal h ole is stressed. The instrument is composed of one electronic unit com manding two identical Sensor Units, each of them allowing to map a ful l hemisphere with a resolution of 1 degrees, thanks to a two-mirrors p eriscope system. The design is described in some details, and the rati onale for choice between several variants are discussed. A hydrogen ab sorption cell is used to measure the shape of the interplanetary Lyman alpha line and other Lyman alpha emissions. Other types of observatio ns are also discussed : the geocorona, comets (old and new), the solar corona, and a possible signature of the heliopause. The connexion wit h some other SOHO instruments, in particular LASCO, UVCS, SUMER, is br iefly discussed.