INTERPLANETARY LYMAN-ALPHA REMOTE-SENSING WITH THE ULYSSES INTERSTELLAR NEUTRAL GAS EXPERIMENT

The Ulysses neutral gas instrument obtained celestial sphere maps of i nterplanetary Lyman ex emission from neutral hydrogen in 1991-1996. Th ese maps are unique because the spacecraft was located at a wide range of heliocentric ecliptic latitudes. Eleven of these maps are compared with the predictions of an interstellar wind hydrogen model previousl y used to study Galileo and Pioneer Venus Lyman alpha data obtained ne ar the ecliptic plane. The model provides reasonable agreement with th e Ulysses maps. The lifetime of the interstellar hydrogen atoms agains t charge exchange with solar wind protons is shown to be more latitudi nally isotropic at solar maximum in 1991 than at solar minimum in 1994 and 1995. This result agrees with white light coronagraph data that g enerally show a more isotropic coronal brightness pattern at solar max imum. The transition from a more symmetric to a more asymmetric solar wind charge exchange rate also explains differences between published Galileo data from solar maximum and Solar and Heliospheric Observatory (SOHO) Solar Wind Anisotropy Experiment (SWAN) data from solar minimu m.