Kinetic Structure of an Interplanetary Shock Observed at Two Heliocentric Longitudes

Collisionless shocks convert bulk flow energy into heat, electromagnetic fields, and non-thermal particle populations. Recent studies suggest that downstream magnetic oscillations could play an important role in ion-scale energy dissipation at low-Mach-number shocks; however, the specific shock and plasma parameters involved remain poorly understood. Interplanetary (IP) shocks, often characterized by low Mach numbers, provide an excellent opportunity for investigating these kinetic dissipation mechanisms. We demonstrate, using observations of an IP shock from the Magnetospheric Multiscale (MMS) and Solar Orbiter (SolO) missions, supported by test-particle simulations, that gyrating protons generate the downstream magnetic oscillations. We found bursts of ion-acoustic waves at the troughs and crests of the magnetic oscillations, suggesting their energy source is related to proton gyration. Comparing MMS and SolO observations, we conclude that the upstream flow speed to ion thermal speed ratio and magnetic compression ratio are key parameters controlling the ion kinetic behavior that produces downstream magnetic oscillations.