The Dynamics of a High Mach Number Quasi-Perpendicular Shock: MMS Observations

TL;DR

This paper analyzes a high Mach number quasi-perpendicular Earth's bow shock using MMS data, revealing nonlinear ion-driven dynamics, shock reformation, and surface ripples in a high plasma beta regime.

Contribution

It provides new observational insights into shock dynamics at high Mach numbers and high plasma beta, a regime less explored by models.

Findings

Reflected ions drive shock perturbations and nonlinear amplification.

Shock reformation occurs but the main shock layer persists.

Surface ripples and proto-shocks are observed.

Abstract

Shock parameters at Earth's bow shock in rare instances can approach the Mach numbers predicted at supernova remnants. We present our analysis of a high Alfv\'en Mach number ($M_A= 27$) shock utilizing multipoint measurements from the Magnetospheric Multiscale (MMS) spacecraft during a crossing of Earth's quasi-perpendicular bow shock. We find that the shock dynamics are mostly driven by reflected ions, perturbations that they generate, and nonlinear amplification of the perturbations. Our analyses show that reflected ions create modest magnetic enhancements upstream of the shock which evolve in a nonlinear manner as they traverse the shock foot. They can transform into proto-shocks that propagate at small angles to the magnetic field and towards the bow shock. The nonstationary bow shock shows signatures of both reformation and surface ripples. Our observations indicate that although shock reformation occurs, the main shock layer never disappears. These observations are at high plasma $\beta$, a parameter regime which has not been well explored by numerical models.