Electron Energization in Quasi-Parallel Shocks: Test-Particle-Electrons in a Proton Driven Turbulence

TL;DR

This study explores how proton-driven turbulence in quasi-parallel shocks can efficiently preheat electrons, challenging the traditional view that electrons are mainly accelerated in quasi-perpendicular shocks, with implications for cosmic ray sources.

Contribution

It introduces a test-particle-electron model within hybrid simulations to demonstrate electron preheating via ion-driven waves in quasi-parallel shocks, highlighting a new acceleration mechanism.

Findings

Proton-driven waves significantly preheat electrons before shock crossing.

Hybrid simulations with increased macro-ions achieve converged electron velocity distributions.

Ion precursor waves can make thermal electrons accessible to acceleration processes.

Abstract

In situ observations of energetic particles at the Earth's bow-shock that are attainable by the satellite missions have long created the opinion that electrons are most efficiently accelerated in a quasi-perpendicular shock geometry. However, shocks that are deemed to be responsible for the production of cosmic ray electrons and their radiation from sources such as supernova remnants are much more powerful and larger than the Earth's bow-shock. Their remote observations and in situ measurements at Saturn's bow shock, suggest that electrons are accelerated very efficiently in the quasi-parallel shocks as well. In this paper we investigate the possibility that protons that are accelerated to high energies create sufficient wave turbulence, which is necessary for the electron preheating and subsequent injection into the diffusive shock acceleration in a quasi-parallel shock geometry. An additional test-particle-electron population, which is meant to be a low-density addition to the electron core-distribution on which the hybrid simulation operates, is introduced. We investigate how these electrons are energized by the hybrid electromagnetic field. The reduced spatial dimensionality allowed us to dramatically increase the number of macro-ions per numerical cell and achieve the converged results for the velocity distributions of test electrons. We discuss the electron preheating mechanisms, which can make a significant part of thermal electrons accessible to the ion-driven waves observed in hybrid simulations. We find that the precursor wave field supplied by ions has a considerable potential to preheat the electrons before they are shocked at the subshock.