The Role of Coulomb Collisions and Large-Scale Electric Field in Shaping Electron Velocity Distributions in the Solar Wind

The solar wind undergoes an extreme change in collisionality from its base outward to one AU and beyond. In the process of this evolution, the electric field, which necessarily exists to lift the ions out of Sun’s gravitational well, goes from a small fraction of the Dreicer critical field to values that are larger than the twice the critical field. Nevertheless, the solar wind remains quasi-neutral and current free. Such a large electric field and the fact that the plasma profiles below the sonic point have Knudsen numbers not much below unity invalidates traditional transport theory and makes it challenging to obtain an accurate description of the overall energy balance in the solar wind. Furthermore, at the radial distances accessible to spacecraft measurements the velocity distribution function (VDF) of the electrons always shows clear deviations from a simple Maxwellian shape which often present as superthermal halo and a field-align Strahl population. In this contribution we describe our progress in describing the global kinetic structure of the solar wind using a model built around 1d drift-kinetic description that includes gravity, a self-consistent radial electric field to ensure quasi-neutrality, as well as a full Landau collision operator for robust description of the Coulomb collisions across the wide range of energies of interest.