Patterns of average potential over the high-latitude ionosphere in winter show that the dusk convection cell dominates the dawn cell, consistent with the presence of a day-night conductivity gradient, as predicted by a number of models. However, in the summer hemisphere, when IMF By is strongly positive, the dusk cell so dominates the dawn cell that the latter nearly disappears; and when IMF By is strongly negative, the cells are most nearly equal. The difference between winter and summer can be explained by the addition in summer of a single lobe cell, that is, a cell confined to open field lines, circulating within the dusk cell of the two-cell pattern when By is positive and within the dawn cell when By is negative. The result is consistent with predictions of the overdraped lobe model, that lobe cells occur in only one hemisphere at a time, and that their occurrence is controlled by dipole tilt. ¿ American Geophysical Union 1993