A simple two-dimensional, steady state, viscous model of the dawnside and duskside low-latitude boundary layer (LLBL) has been developed. It incorporates coupling to the ionosphere via field-aligned currents and associated field-aligned potential drops, governed by a simple conductance law, and it describes boundary layer currents, magnetic fields, and plasma flow in a self-consistent manner. Slab geometry is assumed, with no variations along the flow direction -x and with the layer on closed field lines. The currents in the layer are regulated by coupling to the ionosphere. The magnetic field induced by these currents leads to two effects: (1) a diamagnetic depression of the magnetic field in the equatorial region and (2) bending of the field lines into parabolas in the xz plane with vertices in the equatorial plane, at z=0, and pointing in the flow direction, i.e., tailward. Both effects are strongest at the magnetopause edge of the boundary layer and vanish at the magnetospheric edge. The diamagnetic depression corresponds to an excess of plasma pressure in the equatorial boundary layer near the magnetopause. This pressure drops off both with increasing distance z from the equatorial plane and with increasing distance y from the magnetopause. It reaches the magnetospheric level for z=¿H as well as for y → ∞. The boundary layer structure is governed by a fourth-order, nonlinear, ordinary differential equation in which one nondimensional parameter, the Hartmann number M, appears. A second parameter, introduced via the boundary conditions, is a nondimensional flow velocity v*o at the magnetopause.

It is shown that for large M values the coupling to the ionosphere is weak; in that limit, or when v*o is small, the model reduces to that discussed by lotko et al. (1987) in which induced magnetic fields are neglected. Numerical results from the model are presented and the possible use of observations to determine the model parameters is discussed. The general predictions of the model in terms of region 1 currents and associated ionospheric signatures are similar to those obtained earlier by Lotko et al. and by Sonnerup (1980). Those predictions are in qualitative and approximately quantitative agreement with a number of observations. The main new contribution of the study is to provide a better description of the field and plasma configuration in the LLBL itself and to clarify in quantitative terms the circumstances in which induced magnetic fields become important. In particular, it appears that for the low values of the field-aligned conductance expected on the duskside of the magnetosphere, these fields may remain relatively unimportant, at least in the noon to dusk segment of the LLBL. ¿ American Geophysical Union 1989