Evolution of entropy in the outer heliosphere

Entropy is an important thermodynamic quantity. Based on the conservation equations of a coupled solar wind and turbulence model, and using measurements from Voyager 2, Pioneer 10 and New Horizons, we investigate the solar wind entropy throughout the heliosphere. Observations from Voyager 2, Pioneer 10, and the NH SWAP instrument show different radial profiles of the entropy as a function of heliocentric distance. We find that the entropy measured by (i) Voyager 2 increases by about 12.84% at 75 au, (ii) NH SWAP increases by about 7.8% at 46.37 au, and (iii) Pioneer 10 increases by about 6.5% at 56.82 au. Voyager 2 and NH SWAP made measurements in the upwind direction and Pioneer 10 is in the downwind direction. We compare directly the theoretical results with Voyager 2 measurements only, and find good agreement between them.