Slowdown of the Solar Wind in the Outer Heliosphere and the Interstellar Neutral Hydrogen Density

Voyager 2 and Ulysses were at nearly the same latitude in the southern heliosphere at the beginning of 1999 and their radial separation was about 55 AU. The e ect of pickup ions should be profound at Voyager's position, especially, the deceleration of the solar wind ow. This unique opportunity allows us to investigate the e ect of pickup ions based on direct multi-spacecraft observations. The Ulysses observations were used as input into a 1-dimensional MHD numerical model. We run cases with and without pickup ions in an attempt to estimate the slowdown of the solar wind and the interstellar neutral hydrogen density. The theoretical predictions at the location of Voyager 2 were compared with the observations. For the purpose of this study, we focus on the statistical characteristics rather than detailed structures of the solar wind in the outer heliosphere. We found a decrease of 40 km s 1 or 10% in the radial velocity near 60 AU. This speed decrease implies an interstellar neutral density of 0.05 cm .