Simulating the Solar Wind: The Firehose Instability in the Two-Fluid, Ten-Moment Model

The solar wind is one of the best natural plasmas: it constantly emanates from the sun, expands into and saturates the heliosphere, and interacts with planets including Earth. However, relatively little is known with certainty about the solar wind, including the behavior of pressure and temperature anisotropy instabilities driven by its expansion. We study one such instability—the parallel firehose instability—using the Gkeyll framework's two-fluid, ten-moment model. In our simulation, we include more information about the electrons, including the full electron pressure tensor, to go beyond existing fluid models. To isolate the firehose instability, we initialize the simulation in the firehose unstable regime. We expand upon previous research using the local closure model by applying a novel gradient closure for the heat flux. We present results on comparisons of the simulated growth rate to linear kinetic theory, how the ten-moment limit of the firehose instability saturates, and compare the saturation and nonlinear dynamics to existing nonlinear, hybrid-kinetic simulations.