PIC simulation of the competition of collisional and collisionless processes in the relaxation of the electron velocity distribution function in the young solar wind

As the solar wind expands in the inner heliosphere, the evolution and relaxation of the electron velocity distribution function (eVDF) is governed by a complex interplay of collisional and collisionless processes.
This study investigates with numerical simulations the competition between Coulomb collisions and the electron firehose instability (EFI) - a kinetic instability arising under anisotropic pressure conditions - during the isotropization of the electron VDF.
The goal is to gain deeper insights into how collisional and collisionless processes influence each other in this regime.
Fully kinetic simulations are run using the particle-in-cell (PIC) code OSIRIS, in the presence and absence of Coulomb collisions.
The plasma density  (and hence the collisional frequency) is progressively increased in the presence of collisions to investigate solar wind regions progressively closer to the Sun.