Firehose Instabilities in Solar Wind Plasmas Simulated by a 10-Moment, Multi-Fluid Solver

Pressure anisotropy is a source of free energy observed in a variety of heliospheric and astrophysical plasma systems, including the solar wind. Pressure anisotropy naturally develops from the expansion of the solar wind as it travels out from the Sun, and sufficiently large pressure anisotropies can drive different instabilities that transfer energy. We use the plasma simulation framework Gkeyll and its 10-moment, multi-fluid solver to investigate firehose instabilities, a class of pressure anisotropy-driven instabilities. Specifically, we consider both 1D and 2D systems with a range of plasma betas and anisotropies relevant to space and astrophysical systems. Comparing the linear Vlasov-Maxwell dispersion solution with the linearized 10-moment fluid equation and nonlinear behavior simulated by Gkeyll, we find that this 10-moment model is able to saturate the firehose instability, which has historically eluded fluid solvers. We also investigate the physical impacts of different heat flux closures in the 10-moment model on the evolution of the firehose instability. Utilizing the greater computational efficiency of a fluid solver compared to kinetic or hybrid simulations, we are working towards exploring larger simulation domains to better understand the saturation of pressure anisotropy-driven instabilities, the mechanism of the saturation, and its impacts on the evolution of these instabilities.