The influence of the parallel electric field and Coulomb collisions on the shape of the electron distribution in the solar wind

The interplanetary electric field parallel to the magnetic field plays an essential role in solar wind evolution. In its presence, the strong speed-dependence of Coulomb collisions partitions electron phase space into a low-speed part that remains close to Maxwellian and a collisionless high-speed part. The resulting skewed distribution matches ion density and current, but carries a heat flux. When Knudsen number is of order unity, as is typical the solar wind at 1au, this heat flux can be significantly different from the expressions often assumed (without justification) in fluid models. A recently proposed SERM model introduces a non-perturbative heat flux closure that relies in part on the partition of the phase space. In this contribution, we discuss numerical solutions of the electron drift kinetic equation that include fully nonlinear collision operator and a self-consistent electric field. The results are compared with SERM predictions on the shape of eVDF and the magnitude of the thermal force. Further, we consider a simplified model of global phase-space that includes advection and drag due to collisions, but ignores diffusion. We quantify the impact of collisions on phase-space trajectories as well as the rate at which particles go from the collisionally modified part to the collisionless part of the phase space.