Particle acceleration and wave excitation in quasi-parallel high-Mach-number collisionless shocks: Particle-in-cell simulation

TL;DR

This study uses particle-in-cell simulations to explore how protons and electrons are accelerated in high-Mach-number collisionless shocks, revealing wave generation, shock structure transformation, and efficient particle acceleration mechanisms.

Contribution

It demonstrates the detailed acceleration processes of particles in quasi-parallel shocks, highlighting the role of upstream waves and phase-trapping in particle energization.

Findings

Protons and electrons are accelerated with power-law energy distributions.

Upstream Alfvénic waves influence shock structure, making it resemble a quasi-perpendicular shock.

Proton acceleration is dominated by a Fermi-like process with rapid shock crossings.

Abstract

We herein investigate shock formation and particle acceleration processes for both protons and electrons in a quasi-parallel high-Mach-number collisionless shock through a long-term, large-scale particle-in-cell simulation. We show that both protons and electrons are accelerated in the shock and that these accelerated particles generate large-amplitude Alfv\'{e}nic waves in the upstream region of the shock. After the upstream waves have grown sufficiently, the local structure of the collisionless shock becomes substantially similar to that of a quasi-perpendicular shock due to the large transverse magnetic field of the waves. A fraction of protons are accelerated in the shock with a power-law-like energy distribution. The rate of proton injection to the acceleration process is approximately constant, and in the injection process, the phase-trapping mechanism for the protons by the upstream waves can play an important role. The dominant acceleration process is a Fermi-like process through repeated shock crossings of the protons. This process is a `fast' process in the sense that the time required for most of the accelerated protons to complete one cycle of the acceleration process is much shorter than the diffusion time. A fraction of the electrons is also accelerated by the same mechanism, and have a power-law-like energy distribution. However, the injection does not enter a steady state during the simulation, which may be related to the intermittent activity of the upstream waves. Upstream of the shock, a fraction of the electrons is pre-accelerated before reaching the shock, which may contribute to steady electron injection at a later time.