In this event study, we have compared electric field measurements acquired near magnetic noon during a rocket flight from the SvalRak range with solar wind and interplanetary magnetic field (IMF) observations. The cusp is spatially bifurcated relative to its source regions. The data indicate that many effects observed at northern high latitudes were driven by dayside merging in the Southern Hemisphere, probably near the dawn side of the cusp. Applying the antiparallel merging criterion of Crooker [1979>, we show that complex ground-based optical data are well ordered by considering that incoming interplanetary electric field phase fronts in the solar wind are tilted, allowing the two hemispheres to respond to the same elements of the solar wind stream at significantly different times. The data stream interacts first with the Southern Hemisphere at lag times significantly less than the simple advection time. Northern Hemisphere merging occurred later, the timing separation being related to the tilt and the strong IMF BX. Auroral emissions created by electrons injected from a Southern Hemisphere merging site may be located in close proximity to those from a Northern Hemisphere site, within the same all-sky image. With proper lag times established for the two source regions, it is clear that variations of dayside auroral emissions occur in response to small changes in the interplanetary electric field. The bifurcation is driven by IMF BY, while BX accentuates differences in the timing of the interaction. The detailed harmonization of distinct auroral features with interplanetary drivers strongly supports the utility of the antiparallel merging criterion in estimating when and where the solar wind and the magnetosphere interact. A similar ordering of auroral effects and in situ data with interplanetary variations cannot be achieved if merging proceeded at lower latitudes along a continuous, tilted merging line passing through the subsolar region, as required by the component-merging hypothesis. A consequence of merging limited to the high-latitude regions is that the smaller convection cell is driven by merging in the opposite hemisphere. While these conclusions are based on our analysis of a single interval and need independent confirmation, the investigation opens new possibilities for understanding cusp electrodynamics, implying a much greater solar wind/IMF control of magnetosphere-ionosphere phenomena than previously thought. ¿ 2001 American Geophysical Union