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...

The Dynamics of Very High Alfvén Mach Number Shocks in Space Plasmas

“…Nevertheless, as the simulations are always subject to restrictions on the physical parameters, these findings show the importance of observational validation. Compromises on the mass ratio between the ions and electrons and the ratio between the plasma frequency and the gyrofrequency will affect the physics captured within the simulation environment (Krasnoselskikh et al 2013), such as, for example, instability growth rates. A restriction on the spatial dimensions in the simulation can likewise have an important effect.…”

Shocks in collisionless plasmas

“…Krasnosselskikh et al (2013) recently discussed bow shock observations pointing out that heating mechanisms are not well understood, because the fundamental aspects occur on many different spatial and temporal scales. Numerous other reviews of Earth's bow shock include the articles by Krall (1997), , Bale et al (2005), Eastwood et al (2005), Burgess et al (2012), Treumann (2009), andWilson III (2016).…”

On Electron Adiabaticity in Collisionless Shocks

“…The common way to overcome this problem is to find a compromise for the two parameters that can mostly optimize the purpose of the study. The impact of using low frequency ratio of the electron plasma frequency to gyrofrequency in numerical simulations has been discussed in Krasnoselskikh et al [4]: the electric field is considerably overestimated than it should be for a comparison with the Earth bow shock. Here, I address the question, "What is the consequence of using a smaller frequency ratio upon the adiabatic motion of the electrons"?…”

“…The electric field E can be estimated by using Faraday's law for an electromagnetic whistler wave of frequency ω and wave number k standing on the shock front (as a precursor) and co-moving with the shock velocity v sh , [4], (4) where, M A is the shock Mach number, and V A is the Alfvén velocity,…”