The Interplay Between Ambipolar Electric Field and Coulomb Collisions in the Solar Wind Acceleration Region

The solar wind protons are accelerated to supersonic velocities within the distance of 10 solar radii from the Sun, as a consequence of a complex physical mechanism including particle kinetic effects as well as the field-particle energy and momentum exchange. We use a numerical kinetic model of the solar wind, accounting for Coulomb collisions (BiCoP), and model a solar wind accelerated only by the ambipolar electrostatic filed (E) arising due to the difference in mass between electron and proton, and assuring quasi-neutrality and zero current. We study the effect E, which was found to be on the order of Dreicer electric field (E-D) (Dreicer, 1959), has on the resulting electron velocity distribution functions. The strahl electron radial evolution is represented by means of its pitch-angle width (PAW), and the strahl parallel temperature (T-s,T-||). A continuous transition between collisional and weakly collisional regime results in broader PAW, compared to the single-exobase prediction imposed by the exospheric models. Collisions were found to scatter strahl electrons below 250 eV, which in turn has an effect on the measured T-s,T-||. A slight increase was found in T-s,T-|| with radial distance, and was stronger for the more collisional run. We estimate that the coronal electron temperature inferred from the observations of T-s,T-|| in the solar wind, would be overestimated for between 8% and 15%.