Low-frequency Waves Upstream of Quasi-parallel Shocks: Two-dimensional Hybrid Simulations

In this paper, we study low-frequency waves upstream of quasi-parallel shocks by using two-dimensional (2D) hybrid simulations. Simulation results show that reflected particles can backstream and form a superthermal particle component in plasmas in an area just before the shock front. The component interacts with the incident particles and can result in quasi-parallel and quasi-perpendicular fast magnetosonic waves with comparable wave amplitudes, and they have right-hand and linear polarization, respectively. Further upstream, after being scattered by these upstream waves, the backstreaming particles develop a shell-like velocity distribution so that similar waves can be driven by the free energy from this newly formed distribution, and in this area the quasi-perpendicular waves are dominant over the quasi-parallel ones. Linear theory confirms the generation of these two types of upstream waves.