Dynamics of the structure of electric currents and electrodynamic forces in current sheets
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MA DYNAMICS
Dynamics of the Structure of Electric Currents and Electrodynamic Forces in Current Sheets A. G. Frank and S. N. Satunin Prokhorov General Physics Institute, Russian Academy of Sciences, ul. Vavilova 38, Moscow, 119991 Russia Received March 23, 2011; in final form, April 7, 2011
Abstract—Specific features of the spatial distributions of the electric current and electrodynamic forces in current sheets are examined by studying the magnetic fields in them. It is shown that the j × B forces should lead to a gradual increase in the kinetic energy of the plasma accelerated along the current sheet surface. Exci tation of currents directed oppositely to the main current in the central part of the sheet is observed for the first time, and the time evolution of the forward and reverse currents is investigated. Generation of reversed currents is a manifestation of the dynamic effects caused by the motion of plasma flows in the magnetic field and leading to a change in the magnetic structure of the current sheet. DOI: 10.1134/S1063780X11090066
1. INTRODUCTION It is well known that solar flares release enormous energy previously accumulated in the form of mag netic energy. The released energy is transformed into radiation, thermal and kinetic energy of plasma, and fluxes of accelerated particles, most energy being con verted into the energy of plasma flows [1, 2]. The con cept of current sheets that can form in a magnetized plasma is usually involved as a physical basis of flare phenomena, such as solar and stellar flares, substorms in the magnetospheres of the Earth and other planets, and plasma disruptions in tokamaks [3–8]. The dynamics of current sheets and the processes of mag netic reconnection have been actively studied over the past several decades, including in laboratory experi ments [9, 10]. In many experiments, plasma flows ejected from the sheet with suprathermal velocities were observed [11, 12]. Such flows can be interpreted as an analog of coronal mass ejections (CMEs) [2]. The laboratory experiments conducted under con trolled and well reproducible conditions allow one to compare the characteristics of magnetic fields, electric currents, and electrodynamic forces with the parame ters of plasma flows. Thus, it is interesting to find out what forces accelerate plasma in current sheets and how these forces depend on the sheet formation con ditions, including the characteristic features of the plasma and magnetic field. In this study, the structure of the electrodynamic forces accelerating plasma along the current sheet sur face is thoroughly examined for the first time by inves tigating the spatial distributions of the magnetic fields in the current sheets formed under different condi tions. Specific features of the electric current structure in different stages of the sheet evolution are deter mined, and two new effects that manifest themselves
mainly in the later stages are revealed. It is found that, first, the currents directed oppositely to the main cur rent in the central part of
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