Influence of solar wind parameters on pickup ion beam instabilities upstream of Mars: Linear analysis

Linear instability analyses are performed to investigate the influence of solar wind parameters on instabilities driven by a cool pickup ion beam distribution, which is believed to excite the proton cyclotron waves upstream of Mars. Our analysis reveals that both parallel and oblique waves are excited, with parallel waves showing right-hand polarization and oblique waves exhibiting quasi-perpendicular, quasi-electrostatic characteristics at higher solar wind velocities. The growth rates of both wave types increase with solar wind velocity, while solar wind temperature primarily enhances oblique wave growth, leaving parallel waves unaffected. Quasi-linear theory indicates that parallel waves induce pitch-angle scattering of pickup ions, amplifying wave energy, while oblique waves increase the ion's perpendicular velocity, converting wave energy into ion kinetic energy. These findings advance our understanding of wave-particle interactions and their role in atmospheric escape at Mars.