Mach Number Dependence of Ion-scale Kinetic Instability at Collisionless Perpendicular Shock: Condition for Weibel-dominated Shock

TL;DR

This study explores how ion-scale kinetic instabilities at collisionless perpendicular shocks depend on Mach number, identifying conditions under which Weibel instability dominates, supported by linear theory and PIC simulations.

Contribution

It provides a detailed analysis of the transition from Alfvén-ion-cyclotron to Weibel instability at high Mach numbers using linear and nonlinear simulations.

Findings

Weibel instability dominates at high Mach numbers (>20-40).

Reflected ions behave unmagnetized at strong shocks.

Magnetic fluctuations are smaller than in previous Weibel-dominated shock simulations.

Abstract

We investigate ion-scale kinetic plasma instabilities at the collisionless shock using linear theory and nonlinear Particle-in-Cell (PIC) simulations. We focus on the Alfv\'en-ion-cyclotron (AIC), mirror, and Weibel instabilities, which are all driven unstable by the effective temperature anisotropy induced by the shock-reflected ions within the transition layer of a strictly perpendicular shock. We conduct linear dispersion analysis with a homogeneous plasma model to mimic the shock transition layer by adopting a ring distribution with finite thermal spread to represent the velocity distribution of the reflected ions. We find that, for wave propagation parallel to the ambient magnetic field, the AIC instability at lower Alfv\'en Mach numbers tends to transition to the Weibel instability at higher Alfv\'en Mach numbers. The instability property is, however, also strongly affected by the sound Mach number. We conclude that the instability at a strong shock with Alfv\'en and sound Mach numbers both in excess of $\sim 20{\rm -}40$ may be considered as Weibel-like in the sense that the reflected ions behave essentially unmagnetized. Two-dimensional PIC simulations confirm the linear theory and find that, with typical parameters of young supernova remnant shocks, the ring distribution model produces magnetic fluctuations of the order of the background magnetic field, which is smaller than those observed in previous PIC simulations for Weibel-dominated shocks. This indicates that the assumption of the gyrotropic reflected ion distribution may not be adequate to quantitatively predict nonlinear behaviors of the dynamics in high Mach number shocks.