Thermodynamics of the Magnetotail Current Sheet Thinning

摘要

Substorm growth phase in the magnetotail is characterized by formation of a thin current sheet (CS) becomes unstable due to external or internal drivers. Such instability results in magnetic field line reconnection, the substorm onset. The CS thinning, as a key process of substorm dynamics, has been included into many global and local simulations of the magnetotail magnetic reconnection. However, recent observations indicate that the evolution of plasma characteristics and magnetic field configuration during the CS thinning can differ from predictions of the classical adiabatic scenario. In this study, we combine two most extensive datasets of the CS evolution, as observed by Cluster and THEMIS missions for 2001-2009 and 2015, respectively. We show that for a wide range of downtail distances and dawn-dusk direction there are quite similar quantitative characteristics of the thinning: the magnetic field line stretching (north-south magnetic field decrease), the intensification of the current density, and the evolution of plasma temperatures and densities. We confirm that the process cannot be directly associated with increase of the lobe magnetic pressure. Using advantages of multispacecraft measurements and CS flapping motion, we demonstrate that the thinning is usually result in the equatorial density increase and plasma temperature decrease. We discuss the revealed evolution features in the context of the thermodynamical CS characteristics for contemporary thinning models.