Turbulent Heating of the Solar Wind by Newborn Interstellar Pickup Protons

Spacecraft missions to the outer heliosphere have shown that the thermal protons that make up the solar wind are hotter than simple adiabatic expansion would predict. We examine a theory that describes this heating by using the time-varying 1 AU measurements as input and comparing the predictions with observations in the outer heliosphere. Inside 20 AU wind shear and shocks provide the dominant energy source to drive the turbulence. Outside 20 AU little remains to inject energy into the fluctuations except newborn interstellar pickup protons. The theory is built on a combination of two-dimensional magnetohydrodynamic turbulence concepts and the latest kinetic theory describing the scattering of newborn interstellar pickup protons. We find that application of the theory to the observations produces encouraging agreement at the same time that it illuminates latitudinal effects associated with solar minimum conditions. A remaining challenge is to close the gap of a factor of 2 between observed and predicted proton temperatures beyond 40 AU. For this, we suggest that further structural development of the theory is needed, rather than ad hoc adjustment of the model parameters, which are reasonably well constrained by theory, simulation, and observations.