Thin current sheets in collisionless plasma: Equilibrium structure, plasma instabilities, and particle acceleration
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E PLASMA
Thin Current Sheets in Collisionless Plasma: Equilibrium Structure, Plasma Instabilities, and Particle Acceleration L. M. Zelenyia, H. V. Malovaa, b, A. V. Artemyeva, b, V. Yu. Popovc, a, and A. A. Petrukovicha a b
Space Research Institute, Russian Academy of Sciences, Profsoyuznaya ul. 84/32, Moscow, 117997 Russia Skobeltsyn Institute of Nuclear Physics, Moscow State University, Leninskie gory, Moscow, 119992 Russia c Moscow State University, Moscow, 119992 Russia Received July 15, 2010
Abstract—The review is devoted to plasma structures with an extremely small transverse size, namely, thin current sheets that have been discovered and investigated by spacecraft observations in the Earth’s magneto tail in the last few decades. The formation of current sheets is attributed to complicated dynamic processes occurring in a collisionless space plasma during geomagnetic perturbations and near the magnetic reconnec tion regions. The models that describe thin current structures in the Earth’s magnetotail are reviewed. They are based on the assumption of the quasiadiabatic ion dynamics in a relatively weak magnetic field of the magnetotail neutral sheet, where the ions can become unmagnetized. It is shown that the ion distribution can be represented as a function of the integrals of particle motion—the total energy and quasiadiabatic invari ant. Various modifications of the initial equilibrium are considered that are obtained with allowance for the currents of magnetized electrons, the contribution of oxygen ions, the asymmetry of plasma sources, and the effects related to the nonMaxwellian particle distributions. The theoretical results are compared with the observational data from the Cluster spacecraft mission. Various plasma instabilities developing in thin current sheets are investigated. The evolution of the tearing mode is analyzed, and the parameter range in which the mode can grow are determined. The paradox of complete stabilization of the tearing mode in current sheets with a nonzero normal magnetic field component is thereby resolved based on the quasiadiabatic model. It is shown that, over a wide range of current sheet parameters and the propagation directions of largescale unstable waves, various modified drift instabilities—kink and sausage modes—can develop in the system. Based on the concept of a turbulent electromagnetic field excited as a result of the development and satura tion of unstable waves, a mechanism for charged particle acceleration in turbulent current sheets is proposed and the energy spectra of the accelerated particles are obtained. DOI: 10.1134/S1063780X1102005X
CONTENT 1. INTRODUCTION 2. CURRENT SHEETS 2.1. Theoretical models: Classification and Prop erties 2.2. Models of Particle Motion 2.2.1. QuasiAdiabatic Approximation 2.2.2. Electrons in the Guiding Center Approxima tion 2.3. Simulation of Thin Current Sheets: Basic Equations 2.4. Liouville Theorem: Extension of the Distribu tion Function 2.5. Grad–Shafranov Equation 2.6. Dynamics of Current Sheets 3. SIMULATI
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