Particle-in-cell simulations of particle energization from low Mach number fast mode shocks

Abstract

Astrophysical shocks are often studied in the high Mach number limit but weakly compressive fast shocks can occur in magnetic reconnection outflows and are considered to be a site of particle energization in solar flares. Here we study the microphysics of such perpendicular, low Mach number collisionless shocks using two-dimensional particle-in-cell (PIC) simulations with a reduced ion/electron mass ratio and employ a moving wall boundary method for initial generation of the shock. This moving wall method allows for more control of the shock speed, smaller simulation box sizes, and longer simulation times than the commonly used fixed wall, reflection method of shock formation. Our results, which are independent of the shock formation method, reveal the prevalence shock drift acceleration (SDA) of both electron and ions in a purely perpendicular shock with Alfvén Mach number $M_A=6.8$ and ratio of thermal to magnetic pressure $\beta =8$. We determine the respective minimum energies required for electrons and ions to incur SDA. We derive a theoretical electron distribution via SDA that compares to the simulation results. We also show that a modified two-stream instability due to the incoming and reflecting ions in the shock transition region acts as the mechanism to generate collisionless plasma turbulence that sustains the shock.

References in corpus (2)

• On the structure of relativistic collisionless shocks in electron-ion plasmas
• Nonrelativistic collisionless shocks in weakly magnetized electron--ion plasmas: two-dimensional particle-in-cell simulation of perpendicular shock

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• The Acceleration of Electrons at Collisionless Shocks Moving Through a Turbulent Magnetic Field
• Particle-in-cell simulations of particle energization via shock drift acceleration from low Mach number quasi-perpendicular shocks in solar flares
• Electron Acceleration at Rippled Low-Mach-number Shocks in High-beta Collisionless Cosmic Plasmas
• Electron shock drift acceleration at a low-Mach-number, low-plasma-beta quasi-perpendicular shock
• Proton Acceleration by Collisionless Shocks in Supermassive Black Hole Coronae: Implications for High-Energy Neutrinos