Kinetic Simulation on Electron, Proton and Helium Acceleration in a Nonrelativistic Quasiparallel shock

TL;DR

This study uses 1D PIC simulations to explore how electrons, protons, and helium ions are accelerated in nonrelativistic quasiparallel shocks, revealing efficient particle acceleration and wave excitation relevant to cosmic ray origins.

Contribution

It demonstrates the acceleration of multiple particle species, including heavy ions, in nonrelativistic shocks with self-generated waves, advancing understanding of cosmic ray acceleration mechanisms.

Findings

Waves are excited by energetic protons and helium ions upstream of the shock.

Particles are scattered and gradually accelerated by self-generated waves.

A significant fraction of kinetic energy is transferred to nonthermal particles (7% protons, 5.4% helium).

Abstract

In addition to electrons and protons, nonrelativistic quasiparallel shocks are expected to possess the ability to accelerate heavy ions. The shocks in supernova remnants are generally supposed to be accelerators of the Galactic cosmic rays, which consist of many species of particles. We investigate diffusive shock acceleration (DSA) of electrons, protons and helium ions in a nonrelativistic quasiparallel shock through 1D particle-in-cell (PIC) simulation with a helium-to-proton number density ratio of $0.1$, which is relevant for the Galactic cosmic rays. The simulation indicates that waves can be excited by the flow of the energetic protons and helium ions upstream of the nonrelativistic quasiparallel shock with a sonic Mach number of 14 and an alfv\'{e}n Mach number of 19.5 in the shock rest frame, and the charged particles are scattered by the self-generated waves and accelerated gradually. Moreover, the spectra of the charged particles downstream of the shock are thermal plus a nonthermal tail, and the acceleration is efficient with about $7\%$ and $5.4\%$ of the bulk kinetic energy transferred into the nonthermal protons and helium ions in the near downstream region at the end of the simulation, respectively.