The Heppner-Maynard Boundary (HMB) represents the equatorward extent of the ionospheric convection pattern and can be used as a proxy for the low latitude of the auroral oval. We present a statistical study of the radial interplanetary magnetic field (IMF) effects on the HMB midnight latitude calculated from SuperDARN measurements between January 2002 and December 2017. We found the average values of HMB midnight latitude during both sunward and anti-sunward radial IMF are higher than 65.5º. There is a negative correlation between the magnitude of Bx and HMB midnight latitude, although this effect is not obvious. Moreover, the seasonal variation of Bx-HMB correlation coefficients is different with the existence of single lobe reconnection. At the anti-sunward radial period, the correlation coefficient is up to 0.54 in wintertime. It would be caused by the enhanced lobe reconnection rates, which related to the special configuration between the solar-wind and magnetopause. This is the first long-term statistical study focused on HMB during radial IMF conditions in the context of solar wind-magnetosphere-ionosphere coupling. The results suggest that the effect of IMF Bx should not be ignored in the northern hemisphere wintertime especially during the anti-sunward radial IMF conditions.