Splitting of thin current sheets in the Earth's magnetosphere

An analytic model of a one-dimensional self-consistent anisotropic thin current sheet is proposed. This model describes the sheet with a split (or bifurcated) structure, where the current density is minimal at the center and maximal at the edges. The model is specified by the set of Vlasov-Maxwell equations that reduces to the Grad-Shafranov equation. Under the assumption that particles move quasi-adiabatically, i.e., that the approximate integral of motion I-z is conserved, the slow evolution of the system in the course of diffusion of the distribution function in I-z is analyzed. Scattering processes can give rise to the partial capture of flying ions near the current sheet. Since the current of such quasi-trapped particles is directed oppositely to the current of flying particles, the local current at the center of the sheet is fully or partially compensated. As a result, the ordinary single-peak shape of the current density profile changes to the bifurcated shape. Such a structure is characteristic of the thin current sheet before the total destruction, when the tension of the magnetic field is unbalanced. Numerical calculations are corroborated by the observations of split current sheets in the magnetotail by the Cluster and Geotail satellites. The obtained results indicate that a possible mechanism of the destruction of the thin current sheet is not necessarily associated with the development of plasma instabilities but can be evolutionary. (C) 2003 MAIK Nauka/Interperiodica.