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

• Published in: Journal of geophysical research. Space physics 2021-03, Vol.126 (3), p.n/a
• Main Authors: Berčič, L., Landi, S., Maksimović, M.
• Format: Article
• Language: English
• Subjects: Acceleration; ambipolar electric field; Charged particles; Coronal electrons; Coulomb collisions; Distribution functions; Electric fields; electron distribution function; Electron energy; Electron velocity distribution; Exospheric models; kinetic model; Particle energy; Protons; Sciences of the Universe; Solar wind; Solar wind acceleration; Solar wind protons; Velocity distribution
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2020JA028864

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 (ED) (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 (Ts,‖). 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 Ts,‖. A slight increase was found in Ts,‖ with radial distance, and was stronger for the more collisional run. We estimate that the coronal electron temperature inferred from the observations of Ts,‖ in the solar wind, would be overestimated for between 8% and 15%. Key Points We use a kinetic model of expanding solar wind accounting for Coulomb collisions. This model produces a slow, supersonic solar wind proton population accelerated only through the ambipolar electric field, which arises due to the difference of mass between electron and proton The self‐consistently calculated ambipolar electric field in the model is on the order of Dreicer electric field We present the radial evolution of the strahl electron component under the influence of Coulomb collisions