Influence of the IMF Elevation Angle on Midday Auroral Intensity During Northward IMF

Using global ultraviolet imager auroral observations, we have investigated the possible role of the IMF elevation angle ($\varphi$), which is determined by IMF B_x and B_z, on the midday auroral intensity for northward B_z conditions. The results show that in summer and equinoxes, the peak midday auroral intensity enhances by 11%–19% from non-favorable conditions, when $\varphi$ favors tail lobe reconnection in each hemisphere. Larger absolute and relative difference also occurred in the polar cap, especially in summer. We propose that these $\varphi$ effects contribute to stronger midday auroral intensity due to lobe reconnection, which are associated with special B_x polarities (i.e., B_x < 0 and B_x > 0 in the northern and southern hemispheres, respectively). In addition, when favorable $\varphi$ (or B_x) condition occurs in one hemisphere, we find no evident midday aurora enhancement in the conjugate hemisphere in statistics. This suggests the lobe reconnection generally dominates in one hemisphere for both solstice and equinoctial periods.