Energy Dissipation in Strong Collisionless Shocks: The Crucial Role of Ion-to-Electron Scale Separation in Particle-in-Cell Simulations

Abstract

Energy dissipation in collisionless shocks is a key mechanism in various astrophysical environments. Its non-linear nature complicates analytical understanding and necessitate Particle-in-Cell (PIC) simulations. This study examines the impact of reducing the ion-to-electron mass ratio ($m_r$), to decrease computational cost, on energy partitioning in 1D3V (one spatial and three velocity-space dimensions) PIC simulations of strong, non-relativistic, parallel electron-ion collisionless shocks using the SHARP code. We compare simulations with a reduced mass ratio ($m_r=100$) to those with a realistic mass ratio ($m_r=1836$) for shocks with high (${\mathcal{M}}_A=21.3$) and low (${\mathcal{M}}_A=5.3$) Alfv$\acute{\text{e}}$n Mach numbers. Our findings show that the mass ratio significantly affects particle acceleration and thermal energy dissipation. At high ${\mathcal{M}}_A$, a reduced mass ratio leads to more efficient electron acceleration and an unrealistically high ion flux at higher momentum. At low ${\mathcal{M}}_A$, it causes complete suppression of electron acceleration, whereas the realistic mass ratio enables efficient electron acceleration. The reduced mass ratio also results in excessive electron heating and lower heating in downstream ions at both Mach numbers, with slightly more magnetic field amplification at low ${\mathcal{M}}_A$. Consequently, the electron-to-ion temperature ratio is high at low ${\mathcal{M}}_A$ due to reduced ion heating and remains high at high ${\mathcal{M}}_A$ due to increased electron heating. In contrast, simulations with the realistic $m_r$ show that the ion-to-electron temperature ratio is independent of the upstream magnetic field, a result not observed in reduced $m_r$ simulations.

arXiv comment9 pages, 6 figures; accepted for publication in ApJ Letters (submitted August 3, 2024)

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