OBSERVATIONS OF INTERPLANETARY LYMAN-ALPHA WITH THE GALILEO ULTRAVIOLET SPECTROMETER - MULTIPLE-SCATTERING EFFECTS AT SOLAR MAXIMUM

The Galileo Ultraviolet Spectrometer Experiment (UVS) obtained a parti al celestial sphere map of interplanetary Lyman-alpha (IP Lalpha) on 1 3-14 December 1990 during the first Earth encounter. The Galileo space craft was near the downwind axis of the local interstellar medium flow . These UVS measurements sampled the downwind, anti-sunward hemisphere . The data were modelled using a hot model of the interplanetary hydro gen density distribution with the goal of studying multiple scattering effects in the inner solar system. The derived ratio in the downwind direction of the observed brightness and a single scattering model bri ghtness, both normalized to unity in the upwind direction, is 1.82 +/- 0.2. This brightness ratio requires a multiple scattering correction which is 36% larger than can be accounted for by theoretical calculati ons. Other physical phenomena not included in the model density distri bution must contribute to the downwind brightness enhancement. The hot model may require: (1) a temperature perturbation of the interstellar wind velocity distribution or (2) an additional downstream source of interplanetary hydrogen. However, a more likely explanation which affe cts the hot model is the latitude dependence of the radiation pressure . This dependence, based on the known solar Lalpha flux latitude varia tion at solar maximum, causes a downwind brightness enhancement by pre ferential focusing of H-atoms with trajectory planes containing the so lar poles. This result implies that radiation pressure near the solar poles is nearly independent of solar cycle and is insufficient to lead to a net repulsion of hydrogen atoms by the sun, as can occur near th e ecliptic plane during solar maximum. In addition, the UVS performed 13 observations of IP Lalpha while in cruise between Venus and the Ear th in 3 directions fixed in ecliptic coordinates. The observations mad e while viewing in the ecliptic have more brightness variation in time than observations made viewing ecliptic north, which is a result of e nhanced low-latitude solar activity during solar maximum in 1990. Mode ls of the interstellar wind that fit both the anti-sun map and the UVS 3-position observations require a number density of atomic hydrogen n ear 50 AU of 0.17 +/- 0.05 cm-3.