Lyman alpha emissions in the outer heliosphere from Voyager UVS data

Abstract

Results on the outer heliosphere hydrogen distribution obtained by Baranov and Malama [1993] have been used to estimate the Lyman α intensity backscattered by neutral hydrogen atoms at large distance from the Sun. The existence of the hydrogen wall near the heliopause implies a gradient of hydrogen at large distance from the Sun which significantly affects the Lyman α glow pattern. In these new computations, we have also included variations of local bulk velocity and temperature of the hydrogen distribution in the radiative transfer calculation. Although it has not been possible to find a completely satisfactory fit between the Voyager UVS data presented in Quémerais et al. [1995] and the Baranov-Malama models, we find a better agreement than in the case of the standard hot model, which neglects effects of charge exchange between interstellar protons and hydrogen atoms near the heliopause. On the other hand, the remaining discrepancy between the models and the data, which is observed near the upwind direction, suggests that the intensity excess does not follow the simple axial symmetry used here. Because the directions of the incoming hydrogen atoms and the galactic center nearly coincide, we must alternately consider the possibility that the Lyman α intensity excess may include a component of galactic origin.