[1] Mirror modes are large amplitude nonpropagating compressive structures frequently observed in the magnetosheath. They appear in the form of quasi-sinusoidal oscillations in the magnetic field, profound magnetic decreases (dips) or magnetic enhancements (peaks), accompanied by a corresponding anticorrelated signature in plasma density. In this study we present an analysis of the properties of mirror mode structures in the magnetosheath of the Earth based on a database of Cluster observations and also a detailed case study of one magnetosheath traversal. We focused primarily on the identification of conditions associated with the magnetic dips and magnetic peaks. It is shown that the character of mirror structures is related to the local degree of instability of the plasma with respect to the mirror instability threshold: peaks are typically observed in an unstable plasma, while mirror structures observed deep within the stable region appear almost exclusively as dips. This observation is found to be consistent with recent theoretical and numerical studies. An abrupt transition of mirror structures from peaks to dips at an approximate distance of 2 Earth radii from the magnetopause was identified by multispacecraft analysis and we interpret this effect as a consequence of plasma expansion in the vicinity of the magnetopause locally changing the plasma conditions towards a more stable state.Citation: Soucek, J., E. Lucek, and I. Dandouras (2008), Properties of magnetosheath mirror modes observed by Cluster and their response to changes in plasma parameters,

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...

We present the results of a statistical study of the distribution of mirror and Alfvén‐ion cyclotron (AIC) waves in the magnetosheath together with plasma parameters important for the stability of ULF waves, specifically ion temperature anisotropy and ion beta. Magnetosheath crossings registered by Cluster spacecraft over the course of 2 years served as a basis for the statistics. For each observation we used bow shock, magnetopause, and magnetosheath flow models to identify the relative position of the spacecraft with respect to magnetosheath boundaries and local properties of the upstream shock crossing. A strong dependence of both plasma parameters and mirror/AIC wave occurrence on upstream ΘBn and MA is identified. We analyzed a joint dependence of the same parameters on ΘBn and fractional distance between shock and magnetopause, zenith angle, and length of the flow line. Finally, the occurrence of mirror and AIC modes was compared against the respective instability thresholds. We noted that AIC waves occurred nearly exclusively under mirror stable conditions. This is interpreted in terms of different characters of nonlinear saturation of the two modes.

[1] Magnetic depression structures (magnetic holes) of short time duration from seconds to minutes have been studied using Cluster data in the high-latitude cusp. Our multispacecraft analysis revealed that the magnetic depressions are spatial structures traveling across the spacecraft, and this result was further strengthened by the calculation of the boundary normal directions and velocities using various methods. In this article, we show that multiple properties of the magnetic depressions are consistent with those of mirror structures observed in the magnetosheath or solar wind. The plasma in the cusp is rarely unstable with respect to mirror instability. However, as has been shown by previous studies, once a large magnetic hole is created by mirror instability, it becomes relatively stable and can survive for extended periods of time even if surrounding plasma conditions drop well below the mirror threshold. Although local generation of these structures cannot be completely ruled out in some cases, we propose an interpretation of the magnetic depressions observed in the cusp as mirror structures generated upstream and convected to the cusp by plasma flow. Specifically, the magnetic holes could be generated in the magnetosheath and enter the cusp due to the open geometry of the cusp magnetic field.