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

Two important observed substorm growth phase phenomena are modeled: The formation of a near-Earth (|X ∼ 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_φ ∼ 10 nA/m²) and a half-thickness in the north-south direction of ∼1 R_E, while plasma β is very high (β ∼ 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_∥max ∼ 3 μA/m²) 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.