Nonthermal Acceleration of Electrons, Positrons and Protons at a Nonrelativistic Quasiparallel Collisionless Shock

TL;DR

This study uses particle-in-cell simulations to demonstrate that nonrelativistic quasiparallel collisionless shocks can efficiently accelerate electrons, positrons, and protons, with positrons showing a preferential acceleration pathway due to electric field effects.

Contribution

It provides the first detailed simulation-based analysis of positron acceleration in nonrelativistic shocks, highlighting mechanisms that favor positron injection into diffusive shock acceleration.

Findings

All three species can be accelerated to power-law spectra.

Reflected particles gain significant energy at the shock front.

Positrons are preferentially accelerated due to electric field effects.

Abstract

Energetic positrons have been observed in the interstellar medium, and high-energy positrons with relativistic energies up to approximately 1 TeV have been detected in Galactic cosmic rays. We conducted a study on the acceleration of particles, specifically positrons, in a nonrelativistic quasiparallel collisionless shock induced by a plasma consisting of protons, electrons, and positrons. The positron-to-proton number density ratio in the plasma is 0.1. We focused on a representative shock with a sonic Mach number of 17.1 and an Alfv\'{e}nic Mach number of 16.8 in the rest frame of the shock. To investigate the acceleration mechanisms of particles including positrons in the shock, we utilized one-dimensional particle-in-cell (PIC) simulations. It was found that all three species of particles in the shock can be accelerated and exhibit power law spectra. At the shock front, a significant portion of incoming upstream particles are reflected and undergo significant energy increase, and these reflected particles can be efficiently injected into the process of diffusive shock acceleration (DSA). Moveover, the reflected positrons can be further accelerated by an electric field parallel to the magnetic field when they move along the magnetic field upstream of the shock. As a result, positrons can be preferentially accelerated to be injected in the DSA process compared to electrons.