Thin current sheets in the Earth´s magnetotail

Current sheets are phenomena of general occurrence in space plasmas where they form at the boundaries between plasmas and/or magnetic fields of different origin. Examples beyond the Earth`s magnetospheres are heliospheric and planetary magnetospheric current sheets or coronal filaments. While the global properties of the Earth`s magnetotail current sheet can be well understood as a MHD-scale process resulting from the interaction between the interplanetary magnetic field (IMF) carried by the solar wind and the Earth`s intrinsic field, most dramatic energy conversion processes takes place when thin current sheets are formed with a thickness below several ion inertia lengths. An example is magnetic reconnection, which is one of the key process occuring in a thin current sheet. Here magnetic field energy is converted into particle energy and can affect the large-scale dynamics of the Earth`s magnetotail, leading to explosive energy dissipation during substorms and magnetic storms. The proposed study aims at a comprehensive understanding of thin current sheet processes by examining its properties under different condition, i.e. during rather stable configuration and during active current sheets when reconnection is taking place. Furthermore, the transition phase between quiet and active thin current sheets are studied, which is a more challenging but a fundamental problem in the thin current sheet process. The three key science questions are: (1) How are thin current sheets formed? (2) How does the thin current sheet activate and how does reconnection start? (3) How are thin current sheets structured in the reconnection region? The following three work packages are planned in this research to answer these key questions, based on extensive analysis of data obtained by multi-point observations by Cluster and THEMIS. WP1. Analysis of a stable/quiet thin current sheet WP2. Analysis of transition phase from quiet to disturbed current sheet (reconnection onset) WP3. Analysis of an unstable/active thin current sheet Multi-point data analysis is performed at different spatialemporal scales by combining different approaches of thin current sheet investigation, i.e., stable current sheet analysis and dynamic current sheet analysis. The former analysis allows to understand the slow formation of the thin current sheet and its condition just before the activation, while the latter analysis serves to identify the different local configuration of the active current sheets and their temporal/spatial development in terms of change in the topology as well as resultant particle acceleration processes. The results from these complementary data analysis are compared with the appropriate theoretical models to make further implication on plausible mode of the instabilities or plausible scenario of the evolution of the thin current sheet.

In this project we have studied the current sheet processes in the Earth's magnetotail. Current sheet occurs in space plasmas at boundaries between different plasma and/or magnetic field regions. They are important sites of energy conversion processes having large-scale consequences such as magnetic storms and substorms, which are one of the most important manifestations of the space weather. Among the different current sheet processes magnetic reconnection is an important process for mixing and acceleration of plasmas and is identified throughout the solar system including Earth and different planetary environments, in solar flares as well as in solar wind. Recent multi-spacecraft observations by ESA mission Cluster and NASA mission THEMIS provided ideal data for studying the current sheet in the Earth's magnetotail. The Cluster consists of four spacecraft forming a tetrahedra configuration, which allows to determine gradients in a 3D space. The five THEMIS spacecraft are separated by larger distances, simultaneously covering different regions in the magnetotail current sheet, and enables to study changes of large-scale magnetotail configuration. In this project we used data from these complementary mission to analyze large-scale evolution of the thin current sheet as well as the local activation processes of the current sheets. By applying multi-point data analysis methods, we have obtained evolution of the large-scale thin current sheet from times before it started to activate and during the times of transient and localized changes of the magnetic structures. In particular, we identified the drastic temporal/spatial changes in the magnetic field component normal to the current sheet that redistribute to form a localized thin current sheet region before its activation. Such configuration leads to instabilities causing magnetic reconnection, where the magnetic field energy is converted to particle energy resulting in high-speed plasma flows. Based on detailed event analysis, by reconstructing the current sheet structures, we also found a new type of thin current sheet in the magnetic reconnection events with a weak magnetic field component along the background tail current direction. Furthermore, new types of smallscalemagnetic flux ropes (spiral magnetic field structures) outside the center of the current sheets are identified. These unexpected dynamical 3D signatures obtained in the reconnection current sheets have important implications also for simulation/theoretical work and will serve as an important reference for the future Magnetospheric Multiscale Mission (MMS).