Abstract Transpolar arcs (TPAs) are auroral structures spanning the polar cap. One type appears in conjugate hemispheres and is thought to be located on closed field lines, but how closed flux enters the open polar cap remains debated. We address this using observation and simulation of conjugate TPAs after an interplanetary magnetic field (IMF) By sign reversal. The reversal displaces the reconnection point, redirects magnetospheric convection, and segregates closed field lines that originally mapped to the auroral oval into the lobe. In the ionosphere, this segregation builds new polar caps rooted in the dawnside (Northern Hemisphere) and duskside (Southern Hemisphere) auroral ovals. TPAs form along the old–new polar cap boundary and drift with the new polar cap’s expansion. This expansion replaces the old counterclockwise circulation with a new clockwise circulation in the Northern Hemisphere, and vice versa in the Southern Hemisphere, on the timescale of antisunward IMF propagation.