Energy dissipation in collisionless shocks: MMS observations

Collisionless shocks play a key role in space and astrophysical plasmas, enabling the conversion of large-scale kinetic energy into heat and non-thermal particle populations without relying on binary Coulomb collisions. Instead, these shocks are sustained by collective effects such as wave-particle interactions that are inherently kinetic and often nonlinear. Despite significant observational and theoretical efforts, the precise mechanisms and spatial localization of energy dissipation in collisionless shocks remain debated. While it is widely accepted that dissipation occurs within the shock ramp, simulations and observations have shown that energy conversion may also extend upstream and downstream, involving shock structures such as the foot and overshoot. Observational studies using Magnetospheric Multiscale (MMS) have further highlighted that ion heating are often concentrated in the ramp and foot, while electron heating may remain nearly constant or increase only under specific conditions such as enhanced wave activity in the transition region.

We analyze high-resolution measurements from the MMS mission across multiple quasi-perpendicular bow shock crossings. We quantify energy contribution of different particle species within a vicinity of the shock ramp to analyze energy transfer among electrons, ions and electromagnetic fields within collisionless shocks. By correcting the measured ion and electron distribution functions for instrumental effects, we isolate the energy contributions of each species and examine how they vary throughout the shock structure. We calculate the theoretically expected values for thermal energy from mass conservation principles and Rankine-Hugoniot conditions to analyze the observed deviation from adiabatic behavior in collisionless shocks. Finally, we discuss how energy transfer between species depends on various shock parameters.