A high‐latitude, low‐latitude boundary layer model of the convection current system

Abstract

Observations suggest that both the high‐ and low‐latitude boundary layers contribute to magnetospheric convection, and that their contributions are linked. In the interpretation pursued here, the high‐latitude boundary layer (HBL) generates the voltage while the low‐latitude boundary layer (LBL) generates the current for the part of the convection electric circuit that closes through the ionosphere. However, the interpretation has a self‐consistency problem: in their magnetospheric settings, the two boundary layer generators are ostensibly independent, but in the ionosphere, Ohm's law ties the voltage to the current, and, thereby, couples them. This paper gives a model that joins the high‐ and low‐latitude boundary layers consistently with the ionospheric Ohm's law. It describes an electric circuit linking both boundary layers, the region 1 Birkeland currents, and the ionospheric Pedersen closure currents. The model works by using the convection electric field that the ionosphere receives from the HBL to determine two boundary conditions to the equations that govern viscous LBL'ionosphere coupling. The result provides the needed self‐consistent coupling between the two boundary layers and fully specifies the solution for the viscous LBL‐ionosphere coupling equations. The solution shows that in providing the current required by the ionospheric Ohm's law, the LBL needs only a tenth of the voltage that spans the HBL. (This has led to undervaluing the LBL's role in convection.) The solution also gives the latitude profiles of the ionospheric electric field, parallel currents, and parallel potential. The parallel currents span the convection reversal and shift both north and south relative to the polar cap boundary as the strength of the transpolar potential changes, offering an empirical test of the model. It predicts that the plasma in the inner part of the LBL moves sunward instead of antisunward and that, as the transpolar potential decreases below about 40 kV, reverse polarity (region 0) currents appear at the poleward border of the region 1 currents. A possible problem with the model is its prediction of a thin boundary layer (∼1000 km), whereas thicknesses inferred from satellite data tend to be greater.