Reconstruction of the upstream solar wind at comet 67P

 Rosetta followed comet 67P at heliocentric distances from 1.25 to 3.6 au. The solar wind was observed for much of this time, but significantly deflected and to some extent slowed down by the interaction with the coma. A method is derived to reconstruct the upstream solar wind from H⁺ and He²⁺ observations. The method is based on the assumption that the comet - solar wind interaction can be described by an electric potential that is the same for both H⁺ and He^2+. The reonstructed speed is compared to estimates from the Tao model, as well as OMNI and Mars Express data propagated to the observation point. The reconstruction agrees well with the Tao model for most of the observations, in particular the statistical distribution of solar wind speed. The electrostatic potential relative to the upstream solar wind is derived and shows values from a few tens of V at large heliocentric distances to about 1 kV during solar events and close to perihelion. Reconstructed values of the solar wind for periods of high electrostatic potential are also in good agreement with propagated observations and model results. The Tao model captures some slowing down of high speed streams as compared to observations at Earth or Mars. At low solar wind speeds, below 400 km/s, agreement is better between our reconstruction and Mars observations than with the Tao model. The magnitude of the reconstructed electrostatic potential is a good measure of the slowing down of the solar wind at the observation point.