QUASI-THREE DIMENSIONAL MODELLING OF THE PLASMA SHEET INCLUDING TURBULENCE ON MEDIUM SCALES Elizaveta E. Antonova, Ilya L. Ovchinnikov Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, 119899, Russia ABSTRACT The analysis of electric field and plasma observations in the plasma sheet and auroral zone points to the existence of low frequency turbulence which leads to intensive plasma mixing and equilibration of the electron and ion tempera- tures across the plasma sheet. Particle transport by turbulent electric fields may be the main process of plasma sheet formation. The suggestion about local equality of large-scale convection transport and quasi-diffusive turbulence per- mits to solve the Grad–Shafranov problem and to determine the dependence of the plasma pressure from the magnetic vector potential. The main input parameter of the model is the large-scale distribution of the electrostatic potential. This is computed from the Volland–Stern model for the ionospheric electric field and Tsyganenko’s model of the magnetic field. The plasma distribution across the tail at different geocentric distances is obtained. Its dependence on the interplanetary magnetic field component B z is analyzed. It is shown that if interplanetary magnetic field is northward, the plasma sheet can become convex in the center of the tail which may cause bifurcation of plasma sheet and formation of theta-aurora. INTRODUCTION Observations of bulk velocity in the plasma sheet (Angelopoulos et al., 1992) show that average velocity of plasma in the central plasma sheet is 30–60 km s −1 but its fluctuations are one order of magnitude larger. This may point to the existence of chaotic electric fields and turbulent transport. These results are in good agreement with observations of plasma sheet and auroral electric fields (Maynard et al., 1982; Mozer et al., 1980) where every measurement shows the electric field value to be orders of magnitude higher than the average (i. e. dawn-dusk electric field). Description of the plasma sheet equilibrium and dynamics requires inclusion of turbulent transport. Chaotic plasma motions may lead to particle fluxes opposing against the gradient of particle concentration. In first approximation this can be described as quasi-diffusion with the effective diffusion coefficient much larger than classical. First attempts to analyze the properties of the turbulent flow (Antonova, 1985, 1987) showed that magnetic flux tubes are nonequipotential. Medium scale turbulent electric fields lead to effective mixing and temperature equilibration in the plasma sheet. This conclusion agrees with ion temperature observations in the plasma sheet (Huang and Frank, 1986, 1994) and auroral plasma (Antonova et al., 1991). In this paper we analyze a 1-D model of a turbulent equilibrium current sheet and generalize it to the 3-D case in the tail approximation (i. e. when current sheet thickness is much less than its length). We also investigate the structure of the plasma sheet for different external conditions (i. e. B IMF z and dawn-dusk difference of potentials) and try to obtain relations between the experimentally measured plasma parameters and the theoretical calculations. TURBULENT CURRENT SHEET Plasma transport by turbulent convection vortices can in the first approximation be analysed using an effective dif- fusion coefficient D, which depends on the average amplitude v turb of turbulent velocity pulsations and the corre- lation time τ : D = v 2 turb τ = λ 2 /τ (λ = v turb τ is the characteristic scale of the turbulence). The plasma flux is j = nv − D∇n, where v is the regular (average) velocity, and −D∇n is the diffusive flux opposite to the pressure 1