A massive rotating equatorial plasma sheet dominates Jupiter's magnetosphere and the solar wind and the interplanetary magnetic field (IMF) are not thought to be as important as at Earth. However, in a recent simulation study we found that for a purely northward IMF the Jovian magnetosphere reached an unstable state in which a nearly periodic series of magnetic X and O lines were launched tailward. In this study we have carried out three-dimensional global magnetohydrodynamic simulations to investigate the causes of this dynamic behavior. First, we examined the effects of dynamic pressure on the magnetospheric configuration in the absence of an IMF. We examined a series of northward IMF simulations in which we varied both the IMF magnitude and dynamic pressure. If the outer edge of the rotating plasma sheet is far from the X-line (for large pressure and small IMF), the reconnected flow reaches the dawn magnetopause and exits down the tail. If the neutral line forms closer to the rotation boundary (small pressure and IMF), then the reconnected flux tubes can convect all of the way around Jupiter. When they reach the nightside, they become stretched tailward and can reconnect again. This leads to the periodic behavior. If the neutral line forms very close to the rotation boundary (large pressure and IMF), the Jupiterward flow compresses the rotating plasma sheet. In this case the flow goes around Jupiter but the flux tubes return along the dusk magnetopause and do not participate in reconnection a second time. Copyright 2006 by the American Geophysical Union.