Theory and observations of upward field-aligned currents at the magnetopause boundary layer

The dependence of the upward field-aligned current density (J_‖) at the dayside magnetopause boundary layer is well described by a simple analytic model based on a velocity shear generator. A previous observational survey confirmed that the scaling properties predicted by the analytical model are applicable between 11 and 17 MLT. We utilize the analytic model to predict field-aligned currents using solar wind and ionospheric parameters and compare with direct observations. The calculated and observed parallel currents are in excellent agreement, suggesting that the model may be useful to infer boundary layer structures. However, near noon, where velocity shear is small, the kinetic pressure gradients and thermal currents, which are not included in the model, could make a small but significant contribution to J_‖. Excluding data from noon, our least squares fit returns log(J_{‖,max_cal}) = (0.96 ± 0.04) log(J_{‖_obs}) + (0.03 ± 0.01) where J_{‖,max_cal} = calculated J_‖,max and J_{‖_obs} = observed J_‖.

The dependence of the strength and thickness of field-aligned currents on solar wind and ionospheric parameters

Sheared plasma flows at the low-latitude boundary layer (LLBL) correlate well with early afternoon auroral arcs and upward field-aligned currents. We present a simple analytic model that relates solar wind and ionospheric parameters to the strength and thickness of field-aligned currents (Λ) in a region of sheared velocity, such as the LLBL. We compare the predictions of the model with DMSP observations and find remarkably good scaling of the upward region 1 currents with solar wind and ionospheric parameters in region located at the boundary layer or open field lines at 1100-1700 magnetic local time. We demonstrate that [Formula: see text] and Λ ~ L when Λ/L < 5 where L is the auroral electrostatic scale length. The sheared boundary layer thickness (Δ _m ) is inferred to be around 3000 km, which appears to have weak dependence on V_sw. J_‖ has dependencies on Δ _m , Σ _p , n_sw, and V_sw. The analytic model provides a simple way to organize data and to infer boundary layer structures from ionospheric data.