MMS Observations and Hybrid Simulations of Surface Ripples at a Marginally Quasi‐Parallel Shock

Imogen Gingell,Per‐Arne Lindqvist,Frederick Wilder,Andreas Johlander,James L Burch,Benoit Lavraud,Stephen Fuselier,Katherine A Goodrich,Daniel J Gershman,Robert J Strangeway,Steven J Schwartz,Yuri V Khotyaintsev,Barbara L Giles,David Burgess,Karlheinz Trattner,Hanying Wei,Robert E Ergun,Roy B Torbert,Christopher T Russell

doi:10.1002/2017ja024538

Abstract

AbstractSimulations and observations of collisionless shocks have shown that deviations of the nominal local shock normal orientation, that is, surface waves or ripples, are expected to propagate in the ramp and overshoot of quasi‐perpendicular shocks. Here we identify signatures of a surface ripple propagating during a crossing of Earth's marginally quasi‐parallel (θBn∼45∘) or quasi‐parallel bow shock on 27 November 2015 06:01:44 UTC by the Magnetospheric Multiscale (MMS) mission and determine the ripple's properties using multispacecraft methods. Using two‐dimensional hybrid simulations, we confirm that surface ripples are a feature of marginally quasi‐parallel and quasi‐parallel shocks under the observed solar wind conditions. In addition, since these marginally quasi‐parallel and quasi‐parallel shocks are expected to undergo a cyclic reformation of the shock front, we discuss the impact of multiple sources of nonstationarity on shock structure. Importantly, ripples are shown to be transient phenomena, developing faster than an ion gyroperiod and only during the period of the reformation cycle when a newly developed shock ramp is unaffected by turbulence in the foot. We conclude that the change in properties of the ripple observed by MMS is consistent with the reformation of the shock front over a time scale of an ion gyroperiod.

Highlights

Collisionless shocks are found in many astrophysical plasma environments, such as planetary bow shocks, interplanetary shocks in the solar wind, and supernova remnants
We conclude that the change in properties of the ripple observed by Magnetospheric Multiscale (MMS) is consistent with the reformation of the shock front over a time scale of an ion gyroperiod
For the shock observed by MMS, the ion gyroperiod is tΩ ∼ 13.2s, and we expect the shock to have partially reformed over the 16 s period we observe ion phase space holes

Summary

Introduction

Collisionless shocks are found in many astrophysical plasma environments, such as planetary bow shocks, interplanetary shocks in the solar wind, and supernova remnants. Johlander et al (2016) have utilized high-resolution particle and field data from the Magnetospheric Multiscale (MMS) mission (Burch et al, 2016) to reconstruct the form of a ripple on a relatively fast crossing of the Earth’s bow shock In both cases, the ripple was observed at a quasi-perpendicular bow shock. The MMS mission enables study of the ion and electron phase space at 2 orders of magnitude greater time resolution than the preceeding multispacecraft missions, Cluster and Time History of Events and Macroscale Interactions during Substorms For this reason, it is ideal for the study of nonstationary structure such as ripples and shock reformation. We conclude that the observations are consistent with the occurrence of surface ripples modulated by cyclic reformation of the shock with a period similar to the time taken by the spacecraft to cross the shock

Observations

Simulations

Discussion