The auroral substorm display in the conjugate hemispheres offers a unique tool to understand how the Earth's plasma and magnetic environment respond to changes in the solar wind and the interplanetary magnetic field (IMF). Earlier studies have demonstrated that substorm onset locations in the two hemispheres are systematically displaced due to the orientation of the IMF, but it is still a controversy which IMF parameter is most important. We have analysed more than 6600 substorms identified from global auroral images by Polar UVI from years 1996–2000 plus 2007 and IMAGE FUV from years 2000–2005. We find very strong statistical support for earlier conjugate auroral imaging observations, according to which the IMF clock angle, θc, organizes the average substorm onset locations in both hemispheres. The IMF θc control is a manifestation of dayside/lobe reconnection geometry and magnetic tension on open field lines before tail reconnection resulting in closed field lines with asymmetric footpoints for all θc angles. By organizing the average substorm locations by the IMF By component only we also find statistical significance. The relation is not linear, as reported earlier, but reveals saturation effects that can be explained by the non-uniform penetration of IMF By into the closed magnetosphere.