In this paper we use simultaneous global UV images of the aurora in the two hemispheres to study differences in the polar cap boundary location. We show that the northern and southern auroral ovals circumvent the same amount of magnetic flux, providing additional evidence that the poleward boundary of the aurora coincides with the open/closed field line boundary. During a period of significant flux closure, large asymmetries in the polar cap boundaries developed between the hemispheres. The asymmetry was strongest in the regions where the polar caps contracted the most, suggesting that emerging interhemispheric polar cap asymmetries is an intrinsic phenomenon during substorm expansions, when magnetic flux closes rapidly in the tail. Utilizing the prolonged surveillance of the open/closed boundary location, we show that the growing asymmetries can be accounted for by differences in the ionospheric convection in the two hemispheres. The observations suggest that the differences in convection were due to seasonal differences between the hemispheres, and that the summer hemisphere responded more promptly to changes in magnetospheric convection than the winter hemisphere.