The First Direct Observational Confirmation of Kinematic Collisionless Relaxation in Very Low Mach Number Shocks Near the Earth

Collisionless shocks are ubiquitous throughout the known universe. They mainly convert the energy of the directed ion flow into heating. Upon crossing the shock front, the ion distribution becomes nongyrotropic. Relaxation to gyrotropy then occurs mainly via kinematic collisionless gyrophase mixing and interaction with waves. The theory of collisionless relaxation predicts that the downstream pressure of each ion species varies quasi-periodically with the distance from the shock transition layer and the amplitude of the variations gradually decrease. The oscillations due to each species have their own spatial period and damping scale. Pressure balance requires that the variations in the total plasma pressure should cause anticorrelating variations in the magnetic pressure. This process should occur at all Mach numbers, but its observation is difficult at moderate-/high-Mach numbers. In contrast, such magnetic oscillations have been observed at low Mach number cases of the Venusian bow shock and interplanetary shocks. In this paper, simultaneous in situ magnetic field and plasma measurements from the THEMIS-B and THEMIS-C spacecraft are used to study, for the first time, the anticorrelated total ion and magnetic pressure spatial variations at low-Mach number shocks. It is found that kinematic collisionless relaxation is the dominant process in the formation of the downstream ion distribution and in shaping the downstream magnetic profile of the observed shocks, confirming fundamental theoretical results. Comparison with the results from numerical models allows the role of the different ion species to be investigated and confirms the role heavy ions play in forming the downstream magnetic profile.