A first clear evidence of an effect of the Bx component (in Solar Magnetospheric Coordinates) of the Interplanetary Magnetic Field (IMF) on magnetopause reconnection has been obtained from polar ionospheric electric fields measured with balloons during an away sector of the IMF. This Bx-effect is such that larger electric field values were measured when Bx was small, and vice versa, during intervals of time with Bz?0, and while the transverse component of the IMF (Bt≡√By2+Bz2) and the solar wind speed had approximately constant values. For those cases, changes of Bx form 2 to 8 gammas are found to account for as much as a factor of two decrease in the magnitude of the convection electric field at the polar cap. When Bz>0, there is also some indication that the measured dusk to dawn polar cap electric field (reversed convection) became larger when ‖Bx‖ was small and vice versa.