Simulation study of energy partition in non-relativistic collisionless shocks

Collisionless shocks are common in astrophysical plasmas and are known to be important for the magnetic field amplification and acceleration of both high energy electrons and protons. While diffusive shock acceleration is well established, particle injection into the nonthermal tail remains an important puzzle. In this work we present the results of large-scale one-dimensional particle-in-cell simulations of magnetized, non relativistic, collisionless shocks to discuss how the properties of the injected particles depend on the plasma parameters, namely the Alfvénic Mach numbers and the orientation of the ambient magnetic field with respect to the shock normal. Quasi-parallel and quasi-perpendicular shocks are analyzed. We analyze electron heating and the development of a nonthermal power- law-like tail in the energy spectra, finding that quasiparallel shocks with high Mach number are the most efficient in terms of injection to the highest energies. Reflected particles exchange energy through wave-particle interactions, exciting modes in the upstream and promoting electrons heating. We analyze the nature of these modes and compute the ratio Te/Ti. We analyze individual trajectories of the most energetic particles to discuss how they achieve non-thermal energies.