Hydrodynamic Shock Modifications by the Heat Flux of Non-Thermal Particles

Collisionless plasma shocks are a common feature of many space and astrophysical systems and are sources of high-energy particles and non-thermal emission, channeling as much as 20% of the shock's energy into non-thermal particles. The generation and acceleration of these non-thermal particles have been extensively studied, however, how these particles feedback on the shock hydrodynamics has yet to be fully treated. This work presents the results of self-consistent, hybrid particle-in-cell simulations that show the effect of self-generated non-thermal particle populations on the nature of collisionless, quasi-parallel shocks, which contribute to a significant heat flux upstream of the shock. Non-thermal particles downstream of the shock leak into the upstream region, taking energy away from the shock. This increases the compression ratio, slows the shock, and flattens the non-thermal population's spectral index for lower Mach number shocks. We incorporate this into a revised theory for the jump conditions that include this effect and show excellent agreement with simulations. The results have the potential to explain discrepancies between predictions and observations in a wide range of systems, such as inaccuracies of the predicted arrival times of coronal mass ejections and the conflicting radio and x-ray observations of intracluster shocks. These effects will likely need to be included in fluid modeling to predict shock evolution accurately.