Turbulent heating of the distant solar wind by interstellar pickup protons with a variable solar wind speed

We have been investigating the turbulent heating of the equatorial solar wind, as modeled by a phenomenological description of the turbulent evolution under the action of spherical expansion, shear driving in the inner heliosphere, and interstellar pickup proton driving in the outer heliosphere. This model is able to reproduce the radial trend of solar wind proton temperature observed at the Voyager 2 spacecraft out to nearly 80 AU. In this paper, we extend the model to include the self-consistent slowdown of the solar wind from momentum-loading by the pickup protons. One motivation for this step is to reduce the model temperature beyond 60 AU, where it consistently exceeds the Voyager measurements. However, we find that a decelerating solar wind results in an increased temperature in our model, due to the larger fraction of pickup proton energy driving the turbulence under these circumstances.