Studying effects of Coulomb collisions on the temperatures of solar wind electrons using cylindrical VPIC simulations.

Solar wind plasma expands from the hot solar corona, but its temperature does not decrease as fast as adiabatic expansion would predict. A first-principle kinetic derivation shows that the heating of the solar-wind electrons results from the energy exchange between strahl component and the electrons trapped between the electric potential and magnetic mirror walls (the core) [1]. In this work, we verified the kinetic model by studying the effects of weak Coulomb collisions on the temperature scaling of the isotropic part of the electrons using Cylindrical VPIC simulations. Cylindrical VPIC is a particle-in-cell code with $B_r \propto 1/r$ scaling and scattering rates that can be changed independently, making it perfect for simulating solar wind. In our analysis of electron distribution functions, we found the temperature of trapped electrons increases with the ratio $\nu_{ee}/\nu_{ei}$, higher the $\nu_{ee}/\nu_{ei}$ higher the electron temperatures, implying a strong correlation between the Coulomb collisions and electron temperatures as suggested by the collisional model.