Cross-scale coupling at a perpendicular collisionless shock

TL;DR

This study uses 2D full particle simulations to explore the complex interactions and structure transitions in a perpendicular collisionless shock, highlighting the role of cross-scale coupling between ion and electron dynamics.

Contribution

It demonstrates the transition from cyclic self-reformation to a quasi-stationary shock front and identifies the mechanisms of particle thermalization and wave excitation at different scales.

Findings

Shock structure transitions from self-reformation to stationary.

Electrons and ions thermalized by electromagnetic waves at different frequencies.

Ion acoustic waves facilitate parallel diffusion and cross-scale coupling.

Abstract

A full particle simulation study is carried out on a perpendicular collisionless shock with a relatively low Alfven Mach number (M_A=5). In the present study, we have performed a two-dimensional (2D) electromagnetic full particle simulation with a "shock-rest-frame model". The simulation domain is taken to be larger than the ion inertial length in order to include full kinetics of both electrons and ions. The present simulation result has confirmed the transition of shock structures from the cyclic self-reformation to the quasi-stationary shock front. During the transition, electrons and ions are thermalized in the direction parallel to the shock magnetic field. Ions are thermalized by low-frequency electromagnetic waves (or rippled structures) excited by strong ion temperature anisotropy at the shock foot, while electrons are thermalized by high-frequency electromagnetic waves (or whistler mode waves) excited by electron temperature anisotropy at the shock overshoot. Ion acoustic waves are also excited at the shock overshoot where the electron parallel temperature becomes higher than the ion parallel temperature. We expect that ion acoustic waves are responsible for parallel diffusion of both electrons and ions, and that a cross-scale coupling between an ion-scale mesoscopic instability and an electron-scale microscopic instability is important for structures and dynamics of a collisionless perpendicular shock.