Multiscale physics of particle acceleration in collisionless shocks

Abstract: Shocks in low density plasmas (so-called “collisionless shocks”) are ubiquitous throughout the Universe and are thought to produce nonthermal particles spanning decades of energy (such as cosmic rays). The process of shock acceleration is an intrinsically multiscale problem, connecting plasma microphysics at the shock to self-generated instabilities driven by accelerated particles far from the shock. I will describe the progress in modeling collisionless shock structure and particle acceleration using ab-initio kinetic simulations, focusing on the current understanding of magnetic field amplification mechanisms and the conditions necessary for particle injection into the acceleration process. Our large scale simulations are beginning to show the effects of nonlinear cosmic ray feedback on shock structure and on the resulting cosmic ray spectrum in high-Mach-number shocks. These results help to interpret the emission spectra from young supernova remnants and transient sources.