Near-Earth thin current sheets and Birkeland currents during substorm growth phase

[1] Two important observed substorm growth phase phenomena are modeled: the formation of a near-Earth (\X\ similar to 8 R-E) thin cross-tail current sheet, as well as the equatorward shift of the ionospheric Birkeland currents. Our study is performed by solving the 3D force-balance equation with realistic boundary conditions and pressure distributions. The obtained cross-tail current sheet has large current density (J(phi) similar to 10 nA/m(2)) and a half-thickness in the north-south direction of similar to 1 R-E, while plasma beta is very high (beta similar to 40) in the current sheet region. The Region-1 and Region-2 Birkeland currents, formed on closed field lines due to pressure gradients, move equatorward and become more intense (J(parallel tomax) similar to 3 muA/m(2)) compared to quiet times. The results agree with growth phase observations. We also find that the cross-tail current sheet maps into the ionosphere in the transition region between the Region-1 and Region-2 currents.