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_‖.