The Downwind Solar Wind: Model Comparison with Pioneer 10 Observations

The solar wind in the upwind region has been well modeled using a pickup ion (PUI) mediated MHD model (Zank et al.). It suggests that PUIs have an important role in heating the solar wind in the outer heliosphere. However, the solar wind in the downwind region is not as well understood. Here, we compare the Zank et al. model with Pioneer 10 observations, which allows us to investigate the downwind solar wind out to 60 au. We use a model in which the hydrogen temperature is finite to obtain a proper hydrogen number density distribution in the downwind region and incorporate it into the model. Our results explain Pioneer 10 observations well and indicate that the heating due to PUIs is less effective than in the upwind region since the density of PUIs in the downwind region is less than the upwind PUIs density. We also derive parameters at several possible locations of the downwind termination shock.