The Dynamic Quasiperpendicular Shock: Cluster Discoveries

Abstract

The physics of collisionless shocks is a very broad topic which has been studied for more than five decades. However, there are a number of important issues which remain unresolved. The energy repartition amongst particle populations in quasiperpendicular shocks is a multi-scale process related to the spatial and temporal structure of the electromagnetic fields within the shock layer. The most important processes take place in the close vicinity of the major magnetic transition or ramp region. The distribution of electromagnetic fields in this region determines the characteristics of ion reflection and thus defines the conditions for ion heating and energy dissipation for supercritical shocks and also the region where an important part of electron heating takes place. In other words, the ramp region determines the main characteristics of energy repartition. All of these processes are crucially dependent upon the characteristic spatial scales of the ramp and foot region provided that the shock is stationary. The process of shock formation consists of the steepening of a large amplitude nonlinear wave. At some point in its evolution the steepening is arrested by processes occurring within the shock transition. From the earliest studies of collisionless shocks these processes were identified as nonlinearity, dissipation, and dispersion. Their relative role determines the scales of electric and magnetic fields, and so control the characteristics of processes such as of ion reflection, electron heating and particle acceleration. The determination of the scales of the electric and magnetic field is one of the key issues in the physics of collisionless shocks. Moreover, it is well known that under certain conditions shocks manifest a nonstationary dynamic behaviour called reformation. It was suggested that the transition from stationary to nonstationary quasiperiodic dynamics is related to gradients, e.g. scales of the ramp region and its associated whistler waves that form a precursor wave train. This implies that the ramp region should be considered as the source of these waves. All these questions have been studied making use observations from the Cluster satellites. The Cluster project continues to provide a unique viewpoint from which to study the scales of shocks. During is lifetime the inter-satellite distance between the Cluster satellites has varied from 100 km to 10000 km allowing scientists to use the data best adapted for the given scientific objective.The purpose of this review is to address a subset of unresolved problems in collisionless shock physics from experimental point of view making use multi-point observations onboard Cluster satellites. The problems we address are determination of scales of fields and of a scale of electron heating, identification of energy source of precursor wave train, an estimate of the role of anomalous resistivity in energy dissipation process by means of measuring short scale wave fields, and direct observation of reformation process during one single shock front...

Validation of single spacecraft methods for collisionless shock velocity estimation

“…Finally, the Waves of High frequency and Sounder for Probing of Electron density by Relaxation (WHISPER) experiment [ Dėcrėau et al , ] was used to obtain the local electron plasma frequency in order to estimate the upstream plasma density. The review of Cluster data‐based advances in the physics of quasi‐perpendicular collisionless shocks is given in Krasnoselskikh et al [].…”

Rankine‐Hugoniot Relations in Multispecies Plasma With Gyrotropic Anisotropic Downstream Ion Distributions

“…Within the anisotropic MHD approach, the state equation should be also assumed (Chao & Goldstein, ; Lyu & Kan, ). High Mach number quasi‐perpendicular number shocks are, most probably, nonplanar and/or time dependent (see Krasnoselskikh et al, for a recent review). Quasi‐parallel shocks are widely believed to have a broad, possibly time‐dependent, transition region (see Burgess & Scholer, , for a recent review).…”

Rippled quasi‐perpendicular collisionless shocks: Local and global normals

“…Present computational limitations of PIC codes force to use the set of parameters for which the Alfvén‐to‐light speed ratio is by about two orders of magnitude higher than in the heliosphere. The similarity of the modeled shocks to the observed ones in these very different parameter regimes is yet to be shown [ Krasnoselskikh et al , ].…”