Authors: Jada Walters (University of Arizona), Kris Klein (University of Arizona), James Juno (Princeton Plasma Physics Laboratory), Jason TenBarge (Princeton University), Emily Lichko (University of Chicago)

Pressure anisotropy is a feature observed in a variety of heliospheric and astrophysical plasma systems. In the solar wind, pressure anisotropy can drive different instabilities that transfer energy, and instability thresholds predicted by kinetic linear theory appear to broadly constrain the evolution of anisotropic features as solar wind plasma travels out from the Sun. In this work, 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, in 1D systems with a range of plasma betas and anisotropies relevant to space and astrophysical systems. We find that the 10-moment, multi-fluid solver in Gkeyll is able to saturate the firehose instability, which has historically eluded fluid solvers. We directly compare the linear Vlasov-Maxwell dispersion solution with the linearized 10-moment fluid equation and nonlinear behavior simulated by Gkeyll. We investigate the physical impacts of discrepancies between these different models of the firehose instability. We also study the details of the growth and saturation of firehose instabilities in 1D using Gkeyll’s nonlinear solver. 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.