Using a paraboloid model of the magnetosphere, we compute the potential drop across open field lines as a function of the interplanetary magnetic field strength and direction as well as of solar wind velocity and density. The collisionless conductivity of the magnetosheath plasma near the magnetopause determines the efficiency of the solar wind electric field penetration into the magnetosphere. This reconnection efficiency is generally about 0.1. Thus about one tenth of the total potential produced over the magnetospheric cross section penetrates the polar cap. Knowing the potential drop across the polar cap allows us to determine the strength of the region 1 field-aligned currents as a function of ionospheric conductivity. We compute the magnetic field disturbance produced by the region 1 field-aligned currents by using a simple current loop in which the region 1 field-aligned currents close through ionospheric and magnetopause currents. The region 1 field-aligned currents decrease the magnetic field strength on the dayside of the magnetosphere, moving the cusp to lower latitudes. This corresponds to a sunward displacement of the polar cap and polar oval. The displacement of the polar oval is 8¿ on the dayside and about 3¿ on the nightside when the total strength of the region 1 Birkeland currents is 5 MA. ¿ 2000 American Geophysical Union