Kinetic models of current sheets with a sheared magnetic field

Abstract

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.