Ion Plasma Stability Downstream of Collisionless Shocks Across the Mach Number Regimes

Collisionless shocks, such as planetary bow shocks and interplanetary shocks, can cause a wide range of ion kinetic instabilities in their downstream region. These phenomena (in particular mirror mode, ion cyclotron, and firehose instabilities) are sensitive to upstream solar-wind conditions. Changing the Mach number, from low to high, is expected to modify the balance between wave activity, transient structures, and turbulent fluctuations. However, a systematic comparative picture across Mach number regimes is still lacking.

We investigate the emergence and behaviour of ion kinetic instabilities across shock crossings spanning a broad range of Alfvénic and magnetosonic Mach numbers under selected solar wind conditions. We focus on the role of upstream parameters such as plasma beta, alpha-to-proton abundance ratio, or upstream interplanetary magnetic field fluctuations in shaping downstream instability behaviour. The analysis is based on MMS terrestrial bow shock crossings and cross-checked against interplanetary shocks by Wind and Solar Orbiter, enabling us to disentangle local planetary bow shock effects from more universal shock-driven processes. This study aims to clarify which instabilities dominate under different upstream conditions and how they contribute to plasma variability and energy redistribution downstream of collisionless shocks.