Two‐Dimensional Hybrid Simulations of Compressive Structures Formed During Ion‐Ion Beam Instabilities

Foreshock compressive structures are often linked to the formation of magnetosheath jets—high dynamic pressure structures frequently observed downstream of quasi‐parallel shocks. Formation of these structures is attributed to the nonlinear evolution of foreshock waves excited by ion‐ion beam instabilities. However, the conditions and underlying mechanisms forming these compressive structures remain unclear. In this study, we investigate the evolution of ion‐ion beam instabilities using two‐dimensional hybrid simulations with the continuous injection of ion beams. Our results demonstrate that while quasi‐parallel waves are initially excited, quasi‐perpendicular, compressive, non‐propagating modes—likely formed by nonlinear wave superposition—grow to large amplitudes when the beam ion temperature is sufficiently high, eventually evolving into compressive structures. In contrast, these quasi‐perpendicular modes saturate at small amplitudes, and no significant compressive structures form when the beam ion temperature is insufficient.