[1] In this work we present a new stationary three-dimensional kinetic-fluid code with ions represented by particles and electrons by a massless fluid. The magnetotail current sheet is modeled as a magnetic field reversal with a normal magnetic field component B n . A cross tail electric field E y is included. An ion test particle simulation is performed in these fields assuming an anisotropic plasma source at the magnetospheric lobes, and ion distribution functions with their moments, such as density, current, and temperature are obtained. The plasma is assumed to be quasineutral and we use the electron momentum equation (with m e ! 0 and without collisions) and the continuity equation to derive the electron current density. We set up an iterative technique to obtain a self-consistent solution of our set of equilibrium model equations. We use this code to study how electrons influence the structure of the magnetotail current sheet and how they contribute to the formation of a double peak in the cross-tail current density, which is observed very often by CLUSTER when crossing the neutral sheet. We find that the electron finite Larmor radius term and the electron drift term, are responsible for the formation of a double peak in the total current density even in those case where the ion current density does not display any bifurcated structure. We also obtained an electric field component normal to the current sheet, which is often observed. Another interesting and physically relevant result which we obtained, is the formation of counterstreaming beamlets in the plasma sheet at some distance from the current sheet, which were observed by Galileo spacecraft in 1990. Finally, we compare the simulation results with the most recent CLUSTER observations. Citation: Greco, A., R. De Bartolo, G. Zimbardo, and P. Veltri (2007), A three-dimensional kinetic-fluid numerical code to study the equilibrium structure of the magnetotail: The role of electrons in the formation of the bifurcated current sheet,

We investigate the predictability of solutions of nonlinear relaxation in twodimensional magnetohydrodynamic equations without external forcing terms (free decay). We use a Galerkin approximation for the equations of motion by retaining only three interacting wave vectors, to reduce the system to N = 6 (when the fields have only real amplitudes) or N = 12 (when amplitudes are complex) ordinary differential equations. We found that the asymptotic state of the model with real amplitudes can be predicted, while that of the model with complex amplitudes strongly depends on the time behavior of the phases of fields.

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