Authors: Elliot Johnson (University of Delaware)

Collisional analysis combines the effects of collisional relaxation and large-scale expansion to determine how solar wind parameters evolve as the plasma expands through the heliosphere. Previous studies have applied collisional analysis to the temperature ratio between protons (ionized hydrogen) and α-particles (fully ionized helium), this is the first study to explore the α-proton differential flow with collisional analysis. First, the mathematical model for the collisional analysis of differential flow was derived. Then, in-situ observations from Parker Solar Probe (PSP; r = 0.1 – 0.27 au) were individually applied to this model to generate predictions of the α-proton differential flow values in the near-Earth solar wind. A comparison of these predicted values with contemporaneous measurements from the Wind spacecraft (r = 1 au) shows strong agreement, which suggests that the evolution of differential flow through the inner heliosphere is largely governed by the effects of expansion and Coulomb collisions.