A numerical study of the evolution of the solar wind from Ulysses to Voyager 2

Voyager 2 continues to explore the outer heliosphere as Ulysses studies the latitudinal dependence of the solar wind. During the year 1991 these spacecraft were within 2 degrees latitude and their radial separation was larger than 30 AU. This alignment presents a good opportunity to investigate the evolution of the solar wind and, in particular, the effect of pickup ions, which are an important component of the solar wind in the outer heliosphere. A numerical simulation study is used to model the evolution of solar wind structures from the location of Ulysses (1-5 AU) to Voyager 2 (33-36 AU) in 1891. The Ulysses observations were used as input into the numerical models and the theoretical predictions were compared with the Voyager 2 observations. The model produces profiles and magnitudes of the plasma parameters (e.g., flow speed and density) which are in reasonable agreement with the observations, The inclusion of pickup ions slows the solar wind, reduces the amplitudes of the speed variations and density spikes, and changes the speed of shock propagation. However, within 35 AU, pickup ions do not change the solar wind structures dramatically Our calculations favor a low interstellar neutral hydrogen density, i.e., n(H infinity) = 0.05 cm(-3), for this time period (1991).