Generation of Perpendicular Ion Acoustic Waves in the Ramp Region of Earth's Bow Shock in the Presence of a Flat-Top Electron Velocity Distribution

We investigate the generation and stability of ion acoustic waves (IAWs) in the ramp region of perpendicular Earth's bow shock. Using a fluid model, we derive the dispersion relation of IAWs, assuming a flat-top electron velocity distribution function typically observed at interplanetary and Earth's bow shocks, along with jumps in plasma parameters. Our findings show that these electrostatic modes are non-dispersive within the shock ramp, which is in agreement with in-situ observations from the Magnetospheric Multiscale Mission (MMS). The calculated frequencies and phase velocities align closely with MMS measurements. Furthermore, we find that the growth rate of IAWs is more strongly affected by the ion temperature jump across the shock than by the electron temperature jump. The methodology developed in this work can be extended to investigate the generation and stability of other wave modes in diverse space and astrophysical plasma environments.