Evidence for a solar wind slowdown in the outer heliosphere?

Voyager 2 and IMP 8 plasma data are used to look for the predicted slowdown of the solar wind with heliospheric distance. Decreases of roughly 7% in the radial velocity and of the same order in the flux are found if the Voyager 2 and IMP 8 velocities are normalized to agree in the inner heliosphere. This decrease is consistent with a pickup ion density equal to 8% of the total ion density, similar to predictions and other determinations of this density. Comparison with published model results allows us to infer an interstellar neutral density of 0.05 cm −3 .