Microstructure in two- and three-dimensional hybrid simulations of perpendicular collisionless shocks

TL;DR

This study uses high-resolution 2D and 3D hybrid simulations to analyze the microstructure of perpendicular collisionless shocks, revealing the roles of ion instabilities and coupling between fluctuations and ion motion.

Contribution

First 3D hybrid simulations demonstrate the coupling between field-parallel fluctuations and reflected ion motion as the main source of shock microstructure.

Findings

Ion Weibel instability causes short-wavelength fluctuations in 2D.

Reflected ions drive microstructure in 3D simulations.

Microstructure patterns are inclined to the magnetic field due to coupled fluctuations.

Abstract

Supercritical collisionless perpendicular shocks have an average macrostructure determined primarily by the dynamics of ions specularly reflected at the magnetic ramp. Within the overall macrostructure, instabilities, both linear and nonlinear, generate fluctuations and microstructure. To identify the sources of such microstructure, high-resolution two- and three-dimensional simulations have been carried out using the hybrid method, wherein the ions are treated as particles and the electron response is modelled as a massless fluid. We confirm the results of earlier 2-D simulations showing both field-parallel aligned propagating fluctuations and fluctuations carried by the reflected-gyrating ions. In addition, it is shown that, for 2-D simulations of the shock coplanarity plane, the presence of short-wavelength fluctuations in all magnetic components is associated with the ion Weibel instability driven at the upstream edge of the foot by the reflected-gyrating ions. In 3-D simulations we show for the first time that the dominant microstructure is due to a coupling between field-parallel propagating fluctuations in the ramp and the motion of the reflected ions. This results in a pattern of fluctuations counter-propagating across the surface of the shock at an angle inclined to the magnetic field direction, due to a combination of field-parallel motion at the Alfv\'en speed of the ramp, and motion in the sense of gyration of the reflected ions.