Determining the LIC H density from the solar wind slowdown

Context. The ionized solar wind interacts directly with the interstellar neutrals which flow into the heliosphere. These neutrals are ionized, mainly by charge exchange, then accelerated to the solar wind speed with the momentum and energy removed from the bulk flow of the solar wind. Thus, by measuring the solar wind slowdown, one can estimate the interstellar neutral density. Aims. In July 2005, Ulysses at 5 AU and Voyager 2 near 80 AU were at the same heliolatitude. We use this alignment to determine the solar wind speed decrease between these two spacecraft. Methods. Ulysses data are used as input to a 1-D MHD model which includes the effects of pickup ions. We removed a section of data contaminated by an ICME directed toward Voyager 2. Results. Comparison of the Voyager 2 speeds with the model results shows that the solar wind speed decreased by 67 km s(-1) between Ulysses and Voyager 2, consistent with an interstellar neutral density at the termination shock of 0.09 cm(-3).