Ion acceleration in non-relativistic astrophysical shocks

Abstract

We explore the physics of shock evolution and particle acceleration in non-relativistic collisionless shocks using hybrid simulations. We analyze a wide range of physical parameters relevant to the acceleration of cosmic rays (CRs) in astrophysical shock scenarios. We show that there are fundamental differences between high and low Mach number shocks in terms of the electromagnetic turbulence generated in the pre-shock zone; dominant modes are resonant with the streaming CRs in the low Mach number regime, while both resonant and non-resonant modes are present for high Mach numbers. Energetic power-law tails for ions in the downstream plasma account for up to 15% of the incoming upstream flow energy, distributed over ∼5% of the particles in a power law with slope -2 ± 0.2 in energy. Quasi-parallel shocks with θ ≤ 45° are good ion accelerators, while power laws are greatly suppressed for quasi-perpendicular shocks, θ > 45°. The efficiency of conversion of flow energy into the energy of accelerated particles peaks at θ = 15°-30° and MA = 6, and decreases for higher Mach numbers, down to ∼2% for MA = 31. Accelerated particles are produced by diffusive shock acceleration (DSA) and by shock drift acceleration (SDA) mechanisms, with the SDA contribution to the overall energy gain increasing with magnetic inclination. We also present a direct comparison between hybrid and fully kinetic particle-in-cell results at early times. In supernova remnant (SNR) shocks, particle acceleration will be significant for low Mach number quasi-parallel flows (MA < 30, θ < 45). This finding underscores the need for an effective magnetic amplification mechanism in SNR shocks.