Kinetic models of current sheets with a sheared magnetic field

Thin current sheets, whose existence in the Earth’s magnetotail is confirmed by numerous spacecraft measurements, are studied analytically and numerically. The thickness of such sheets is on the order of the ion Larmor radius, and the normal component of the magnetic field (B _z ) in the sheet is almost constant, while the tangential (B _x ) and shear (B _y ) components depend on the transverse coordinate z. The current density in the sheet also has two self-consistent components (j _x and j _y , respectively), and the magnetic field lines are deformed and do not lie in a single plane. To study such quasi-one-dimensional current configurations, two kinetic models are used, in particular, a numerical model based on the particle-in-cell method and an analytical model. The calculated results show that two different modes of the self-consistent shear magnetic field B _y and, accordingly, two thin current sheet configurations can exist for the same input parameters. For the mode with an antisymmetric z profile of the B _y component, the magnetic field lines within the sheet are twisted, whereas the profiles of the plasma density, current density component j _y , and magnetic field component B _x differ slightly from those in the case of a shearless magnetic field (B _y = 0). For the symmetric B _y mode, the magnetic field lines lie in a curved surface. In this case, the plasma density in the sheet varies slightly and the current sheet is two times thicker. Analysis of the dependence of the current sheet structure on the flow anisotropy shows that the sheet thickness decreases significantly with decreasing ratio between the thermal and drift plasma velocities, which is caused by the dynamics of quasi-adiabatic ions. It is shown that the results of the analytical and numerical models are in good agreement. The problems of application of these models to describe current sheets at the magnetopause and near magnetic reconnection regions are discussed.     

Albert Galeev: The Problem of Metastability and Explosive Reconnection

Plasma Physics Reports - −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...     

Drift modes of a quasi-two-dimensional current sheet

Stability of a plasma configuration consisting of a thin one-dimensional current sheet embedded into a two-dimensional background current sheet is studied. Drift modes developing in plasma as unstable waves along the current direction are considered. Dispersion relations for kink and sausage perturbation modes are obtained depending on the ratio of parameters of thin and background current sheets. It is shown that the existence of the background sheet results in a decrease in the instability growth rates and a significant increase in the perturbation wavelengths. The role of drift modes in the excitation of oscillations observed in the current sheet of the Earth’s magnetotail is discussed.     

System of kinetic equations describing large-scale processes in collisionless space plasma

A system of kinetic equations describing relatively slow large-scale processes in collisionless magnetoplasma structures with a spatial resolution of about the characteristic gyroradius is derived. Plasma is assumed to be quasineutral, while the magnetic and electric fields are determined by the instantaneous distributions of the particle and current densities and the stress tensor of all plasma components in the longrange instantaneous interaction approximation. A special version of equations is derived for the case of magnetized electrons described by the Vlasov equation in the drift approximation. The obtained system of equations can be used to develop a global numerical kinetic model of the Earth’s magnetosphere with a spatial resolution of about 100 km, as well as local models of certain regions of the Earth’s magnetosphere with a higher resolution.     

Quasi-adiabatic dynamics of ions in a bifurcated current sheet

The study is devoted to ion dynamics in bifurcated current sheets with a two-peak current-density distribution observed in the Earth’s magnetotail and solar wind. The ion motion is described by a Hamiltonian system with two degrees of freedom. The presence of a small parameter κ characterizing the ratio between the amplitudes of the normal and tangential magnetic field components allows one to separate variables into fast and slow ones and introduce the quasi-adiabatic invariant of motion I _z . Conservation of this invariant makes it possible to analytically describe the dynamics of charged particles. Deviations of the particle dynamics from the quasi-adiabatic one, which are caused by the nonconservation of the quasi-adiabatic invariant, are investigated. The jump of the invariant ΔI _z is shown to depend on the small parameter according to the power-law ΔI _z ～ κ ^h , where the exponent h varies between unity and 3/4, depending on the level of current sheet bifurcation. The obtained dependence of ΔI _z on κ coincides with analytic expressions in the limiting cases of nonbifurcated and completely bifurcated current sheets.     

Role of Oxygen Ions in the Structure of the Current Sheet of the Near-Earth Magnetotail

Plasma Physics Reports - A numerical model is used to study the possibility of a thin current sheet formation in the near-Earth magnetotail in the growth phase of a substorm for a wide range of...     

Asymmetric configurations of a thin current sheet with a constant normal magnetic field component

A possible mechanism for the formation of a quasi-equilibrium asymmetric current sheet in the magnetospheric tail due to the asymmetry of peripheral plasma sources is analyzed using a self-consistent particle- in-cell model of a thin collisionless current sheet with a constant normal magnetic field component. For the case in which the current sheet is produced by only one source, quasi-equilibrium sheet configurations with maximum possible asymmetry are obtained for different input parameters of the model. In such configurations, the equilibrium force balance is satisfied with high accuracy and the shape of the current density profile remains nearly symmetric, but the current sheet itself is slightly shifted from the source as compared to the symmetric case. The configurations obtained using numerical simulations are compared with those calculated using the previous analytical model of a thin current sheet. It is found that the results provided by these models agree well both qualitatively and quantitatively.     

On the Role of Magnetic Fields in the Plasma of Dusty Lunar Exosphere

Plasma Physics Reports - The possible effect is studied of the magnetic field of Earth’s magnetotail and the magnetic field in the regions of magnetic anomalies of the Moon on the processes...     

Dusty Plasma at the Moon. Challenges of Modeling and Measurements

Plasma Physics Reports - A brief review is given of the dusty plasma near the surface of the Moon. Electrostatic processes represent an important factor of formation of such plasma. Problems...