The large-scale convection electric field results from the interaction between the solar wind and the outer Earth's magnetosphere and is modulated by the coupling between the ionosphere and the magnetosphere in the auroral and subauroral regions. The corresponding electrostatic potential distribution is basically controlled by two parameters: the cross-polar cap potential drop imposed by the merging of solar wind and magnetospheric plasmas and magnetic fields and the latitudinal attenuation rate (shielding effect) of this potential, toward lower latitudes, by the coupling between the magnetospheric ring current and the conductive ionosphere, along the nearly equipotential geomagnetic field lines. The semiempirical Stern--Volland model, which assumes a two-cell convection pattern, gives a very simple parametrization of these effects, and it is used in the present study as the starting point for the development of a self-consistent model of the large-scale convection electric field. We derive what we have called the modified variable phase Stern--Volland model which depends on the same parameters but includes the latitudinal variation of the local time of the maximum electrostatic potential, required by the self-consistent treatment of the coupling between the magnetospheric and ionospheric transports. We also discuss the connection between the latitudinal attenuation rate of the potential and the resonant wave-particle interactions at harmonics of the ion drift frequency in the ring current.