Electron Influence on the Parallel Proton Firehose Instability in 10-Moment, Multi-Fluid Simulations

Instabilities driven by pressure anisotropy play a critical role in modulating the energy transfer in space and astrophysical plasmas. For the first time, we simulate the evolution and saturation of the parallel proton firehose instability using a multi-fluid model without adding artificial viscosity. These simulations are performed using a 10-moment, multi-fluid model with local and gradient relaxation heat-flux closures in high-$\beta$ proton-electron plasmas. When these higher-order moments are included and pressure anisotropy is permitted to develop in all species, we find that the electrons have a significant impact on the saturation of the parallel proton firehose instability, modulating the proton pressure anisotropy as the instability saturates. Even for lower $\beta$s more relevant to heliospheric plasmas, we observe a pronounced electron energization in simulations using the gradient relaxation closure. Our results indicate that resolving the electron pressure anisotropy is important to correctly describe the behavior of multi-species plasma systems.