Quasiperpendicular high Mach number Shocks

TL;DR

This study presents in situ observations of high Mach number quasi-perpendicular shocks, revealing cyclic reformation driven by ion reflection and its dependence on Mach number, bridging terrestrial and astrophysical shock understanding.

Contribution

First observational evidence of cyclic reformation at very high Mach number shocks using Cassini data, linking microphysics to large-scale shock behavior.

Findings

Reformation occurs at ~0.3 ion gyroperiod timescale.

Reforming shocks show stronger magnetic field amplification.

Reformation is controlled by specular ion reflection.

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 quasi-perpendicular shocks across two orders of magnitude in Alfven Mach number (MA) 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 timescale of ~0.3 {\tau}c, where {\tau}c is the ion gyroperiod. In addition, we experimentally reveal the relationship between reformation and MA and focus on the magnetic structure of such shocks to further show that for the same MA, a reforming shock exhibits stronger magnetic field amplification than a shock that is not reforming.