Understanding particle acceleration in collisionless shocks using kinetic simulations

Princeton University, Princeton NJ

Investigators

Abstract

Collisionless shocks occur in tenuous plasmas where the mean free path for collisions mediated by electrostatic interaction is large, as found in supernova remnants, clusters of galaxies, relativistic jets, and pulsar winds. The conditions under which this occurs, and the efficiency of accelerating charged particles, are not well understood. This project will carry out advanced numerical simulations and use them to construct a self-consistent theory of shock acceleration. It will resolve the complex microphysics of plasma instabilities, particle scattering, and magnetic field generation and amplification, and calibrate the theory for its application to astrophysical objects. The results will help to interpret new studies of high-energy astrophysical and cosmological sources, including the next generation of laboratory experiments, and the physical insights will even apply to shocks in our solar system. The work will train graduate and undergraduate students in numerical modeling and the visualization of multiscale systems, mentor a postdoc, and support open source simulation software and its analysis tools. Collisionless shocks span different speeds and strengths of magnetic field, and accelerate both nonthermal electrons which energize synchrotron emission, and the protons commonly found in cosmic rays. The main process is thought to be diffusive shock acceleration, in which particles scatter on magnetic fluctuations, but the conditions for this Fermi mechanism and its efficiency are not understood from first principles. This project will carry out large-scale multidimensional particle-in-cell (PIC) and hybrid simulations of astrophysical collisionless shocks to consider: 1) what is the relative efficiency of acceleration as a function of shock speed, Mach number, and magnetic inclination? 2) what waves mediate electron acceleration in quasi-parallel and quasi-perpendicular shocks? 3) what is the ratio of electron to ion temperature downstream, and what is the physics of energy transfer? 4) what is the nonlinear structure of the shock when ion acceleration is efficient? With microphysical parameterizations measured with PIC simulations, new software will combine a magneto-hydrodynamic description of the background plasma with kinetic particles to represent cosmic rays and their back-reaction on the flow. This will enable modeling of macroscopic astrophysical objects such as supernova remnants or galactic outflows, including the effects of cosmic rays and their acceleration. These kinetic simulations address how nonlinear shock acceleration works and will constrain the parameters of the flow when it is efficient. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.