10-Moment, Multi-Fluid Simulations of Proton Firehose Instabilities with Electron Dynamics

In weakly collisional solar and astrophysical plasmas, pressure anisotropy-driven instabilities such as the firehose instabilities are common. Simulating proton-electron plasmas to study pressure anisotropy-driven instabilities is computationally expensive for fully kinetic models, and hybrid models typically simplify the electron species to an isothermal, massless neutralizing fluid. Using the plasma simulation framework Gkeyll, we run high-resolution simulations of firehose instabilities with a 10-moment, multi-fluid model. The higher order moments and gradient relaxation closure contained in this model permit pressure anisotropy to develop in all species. Allowing a finite anisotropy enables the electrons to play a significant role in the saturation of the parallel proton firehose instability. We present an expanding box extension to the 10-moment model to investigate how the additional free energy present in expanding plasma systems affects the evolution and saturation of firehose instabilities.