Interplanetary Lyman α remote sensing with the Ulysses Interstellar Neutral Gas Experiment

Abstract

The Ulysses neutral gas instrument obtained celestial sphere maps of interplanetary Lyman α emission from neutral hydrogen in 1991–1996. These 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 previously used to study Galileo and Pioneer Venus Lyman α data obtained near the ecliptic plane. The model provides reasonable agreement with the Ulysses maps. The lifetime of the interstellar hydrogen atoms against charge exchange with solar wind protons is shown to be more latitudinally isotropic at solar maximum in 1991 than at solar minimum in 1994 and 1995. This result agrees with white light coronagraph data that generally show a more isotropic coronal brightness pattern at solar maximum. 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 minimum.