The role of three-dimensional effects on ion injection and acceleration in perpendicular shocks

TL;DR

This study uses 2D and 3D hybrid simulations to investigate ion injection and acceleration at perpendicular shocks, highlighting the critical role of three-dimensional turbulence and resolution in enabling efficient cosmic ray acceleration.

Contribution

It demonstrates that accurate modeling of ion injection and acceleration requires high-resolution 3D simulations to capture turbulence effects.

Findings

Efficient ion acceleration occurs only in 3D simulations.

Ion injection depends on the 'porosity' of downstream magnetic turbulence.

Small-scale turbulence resolution is crucial for accurate particle energization.

Abstract

Understanding the conditions that enable particle acceleration at non-relativistic collisionless shocks is essential to unveil the origin of cosmic rays. We employ 2D and 3D hybrid simulations (with kinetic ions and fluid electrons) to explore particle acceleration and magnetic field amplification in non-relativistic perpendicular shocks, focusing on the role of shock drift acceleration and its dependence on the shock Mach number. We perform an analysis of the ion injection process and demonstrate why efficient acceleration is only observed in 3D. In particular, we show that ion injection critically depends on the "porosity" of the magnetic turbulence in the downstream region near the shock, a property describing how easily the post-shock region allows particles to traverse it and return upstream without being trapped. This effect can only be properly captured in 3D. Additionally, we explore the impact of numerical resolution on ion energization, highlighting how resolving small-scale turbulence -- on scales below the thermal ion gyroradius -- is essential for accurately modeling particle injection. Overall, our results emphasize the necessity of high-resolution 3D simulations to capture the fundamental microphysics driving particle acceleration at perpendicular shocks.