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Albert Galeev: The Problem of Metastability and Explosive Reconnection
−Albert Abubakirovich Galeev is a Soviet and Russian expert in plasma physics who actively contributed to fusion research. In the early 1970s, he became a head of department at the Space Research Institute of the Academy of Sciences of USSR and began devoting most of his time to the problems of the physics of space plasma and made a very important contribution to the solution of many of them, such as physics of collisionless shock waves, the phenomenon of anomalous ionization, processes in the plasma envelopes of comets, and many others. This paper is devoted to only one of the many directions of his work: studies of current sheets and the magnetic reconnection processes that occur therein. Studies of thin current structures is space plasma, whose thickness is about the proton gyroradius, began with the pioneering works of S.I. Syrovatskii, T. Speiser, and other outstanding scientists who proposed that in space plasma, thin boundary current sheets exist, which play the key role in the dynamics of Earth’s magnetosphere and Sun’s corona. The development of these works was dictated by the necessity to explain the solar flares and magnetospheric perturbations during which phases of evolutionary development are replaced by explosive spontaneous processes that release free energy. One of the key physical processes is the magnetic field reconnection, which is realized in nature as a part of the general problem of generation and evolution of current sheets. In a series of works that started in 1975 by the publication (together with L.M. Zelenyi) of the article entitled “Metastable states of a diffuse neutral layer” in JETP letters, A.A. Galeev studied the stability of current sheets to the tearing mode and the dynamics of magnetic reconnection at the boundary of planetary magnetospheres and explained the processes of generation of fast ion flows with energies of several MeV in Earth’s magnetotail. In this paper, we discuss further development of these works that were once initiated by A.A. Galeev. A new model of embedded current sheets is presented, which consists of an internal electron sheet and two external current sheets formed by proton and oxygen ion currents. It is shown that the free energy of such embedded structure in the corresponding range of parameters substantially exceeds the free energy of the well-known Harris’s configuration. This allows one to simultaneously explain their stability (up to a certain limit) and destabilization when the current sheet parameters reach certain critical values, which leads to the change of topology of magnetic field and start of magnetic reconnection.