Quasiperpendicular High Mach Number Shocks.

A H Sulaiman,M Fujimoto,M K Dougherty,A Masters,G B Hospodarsky,D Burgess

doi:10.1103/physrevlett.115.125001

Abstract

Shock waves exist throughout the Universe and are fundamental to understanding the nature of collisionless plasmas. Reformation is a process, driven by microphysics, which typically occurs at high Mach number supercritical shocks. While ongoing studies have investigated this process extensively both theoretically and via simulations, their observations remain few and far between. In this Letter we present a study of very high Mach number shocks in a parameter space that has been poorly explored and we identify reformation using in situ magnetic field observations from the Cassini spacecraft at 10 AU. This has given us an insight into quasiperpendicular shocks across 2 orders of magnitude in Alfvén Mach number (M_{A}) which could potentially bridge the gap between modest terrestrial shocks and more exotic astrophysical shocks. For the first time, we show evidence for cyclic reformation controlled by specular ion reflection occurring at the predicted time scale of ~0.3τ_{c}, where τ_{c} is the ion gyroperiod. In addition, we experimentally reveal the relationship between reformation and M_{A} and focus on the magnetic structure of such shocks to further show that for the same M_{A}, a reforming shock exhibits stronger magnetic field amplification than a shock that is not reforming.

Highlights

In this Letter we present a study of very high Mach number shocks in a parameter space that has been poorly explored and we identify reformation using in situ magnetic field observations from the Cassini spacecraft at 10 AU
This has given us an insight into quasiperpendicular shocks across 2 orders of magnitude in Alfvén Mach number (MA) which could potentially bridge the gap between modest terrestrial shocks and more exotic astrophysical shocks
The interactions depend in complex ways on the shock parameters such as the Mach number M, the ratio of the relative flow speed to a characteristic wave speed, the upstream plasma beta β, the ratio of thermal to magnetic pressures, and θBn, the angle between the upstream magnetic field vector B and the normal to the shock front n

Summary

Introduction

In this Letter we present a study of very high Mach number shocks in a parameter space that has been poorly explored and we identify reformation using in situ magnetic field observations from the Cassini spacecraft at 10 AU. At such high Mach number shocks, the structure becomes inherently dependent on the ion dynamics, and a fluid description is inadequate.

Results

Conclusion