IMF control of the theta aurora and substorm onset location in the conjugate hemispheres

Based on simultaneous imaging data from IMAGE FUV and Polar VIS Earth Camera, we examine the IMF control of the occurrence and location of auroral features in the conjugate hemispheres. The imaging data have revealed that transpolar arcs during northward IMF can exist in one hemisphere but not in the other. We attribute this occurrence of non-conjugate theta aurora to the sign of the IMF B_x component, which controls the hemisphere in which lobe reconnection is favored. Examining auroral substorm onsets in the conjugate hemispheres, we find that the onset locations are displaced in one hemisphere compared to the other, meaning they have different geomagnetic coordinates (e.g., apex, CGM). We find that the relative displacement of onset locations in the conjugate hemispheres are controlled by ratio of B_y/B_z or the IMF clock-angle. These results indicate that the magnetic tensions force is acting on open magnetic field lines before reconnecting in the magnetotail. This leads to a twisted magnetotail and can explain why conjugate field lines have different geomagnetic coordinates in the northern and southern hemispheres.