Timing mirror structures observed by Cluster with a magnetosheath flow model

V Génot,P M Trávníček,P Hellinger,L Broussillou,E Budnik,I Dandouras,E Lucek

doi:10.5194/angeo-29-1849-2011

Abstract

Abstract. The evolution of structures associated with mirror modes during their flow in the Earth's magnetosheath is studied. The fact that the related magnetic fluctuations can take distinct shapes, from deep holes to high peaks, has been assessed in previous works on the observational, modeling and numerical points of view. In this paper we present an analytical model for the flow lines and velocity magnitude inside the magnetosheath. This model is used to interpret almost 10 years of Cluster observations of mirror structures: by back tracking each isolated observation to the shock, the "age", or flow time, of these structures is determined together with the geometry of the shock. Using this flow time the evolutionary path of the structures may be studied with respect to different quantities: the distance to mirror threshold, the amplitude of mirror fluctuations and the skewness of the magnetic amplitude distribution as a marker of the shape of the structures. These behaviours are confronted to numerical simulations which confirm the dynamical perspective gained from the association of the statistical analysis and the analytical model: magnetic peaks are mostly formed just behind the shock and are quickly overwhelmed by magnetic holes as the plasma conditions get more mirror stable. The amplitude of the fluctuations are found to saturate before the skewness vanishes, i.e. when both structures quantitatively balance each other, which typically occurs after a flow time of 100–200 s in the Earth's magnetosheath. Comparison with other astrophysical contexts is discussed.

Highlights

Mirror modes have been studied extensively in recent years both by theoretical and observational means
Mirror modes are mostly studied in the framework of the mirror instability which develops in anisotropic plasma (T⊥/T > 1) with large β alternative views exist (Stasiewicz, 2004)
In this paper it was shown how to build and use a simple analytic model of magnetosheath magnetic and velocity fields in order to obtain the timing of mirror structures, i.e. their “age” since their formation

Summary

Introduction

Mirror modes have been studied extensively in recent years both by theoretical and observational means. Despite this activity a global physical description is still lacking. Mirror modes are mostly studied in the framework of the mirror instability which develops in anisotropic plasma (T⊥/T > 1) with large β alternative views exist (Stasiewicz, 2004). Mirror structures evolve significantly during their travel from the bow shock, where the instability is supposedly triggered, to the depletion layer close to the magnetopause or flank magnetosheath. In the following we propose to infer the history of observed plasma structures from an analytical magnetosheath model. By assuming that mirror structures are created at, or close to the shock, we use this model to gain an understanding of the structure evolution

Objectives

Results

Conclusion