In this paper we present observations of long-period magnetospheric-ionospheric perturbations during northward interplanetary magnetic field (IMF). On November 10--11, 1998, the IMF was northward for 29 hours. The solar wind and IMF parameters were relatively steady. After the IMF had been northward for 14 hours, strong ionospheric velocity (or electric field) perturbations were observed by the Iceland West HF radar in the postmidnight/dawn sector. The velocity perturbations showed periods in the range 50--60 min, with a mean value ~54 min. The Super Dual Auroral Radar Network observed periodic changes of nightside ionospheric convection. For each cycle a large-scale convection cell formed around 0200 magnetic local time (MLT) near magnetic latitude 70¿ and grew for ~30 min. The convection cell then moved eastward with a mean velocity of ~1.4 km s-1. The final position of the cell focus was around 0600 MLT near magnetic latitude 77¿. Ground magnetometers recorded weak magnetic perturbations with periods 50--60 min in both the premidnight and postmidnight sectors. The GOES 8 satellite also observed magnetospheric magnetic field perturbations with similar periods at L~6 on the nightside. The satellite and the key radars which observed the convection oscillations are nearly magnetically conjugate, so the good temporal correlation between the satellite and radar data indicates that the magnetospheric and ionospheric perturbations have a common source. We propose that the magnetospheric-ionospheric perturbations originated within the magnetosphere. The transformation of the magnetospheric tail from an extended to a more dipolar shape during northward IMF is associated with 40--60 min period global tail oscillations which ultimately result in the generation of field-aligned currents and nightside ionospheric convection vortices near X=-10 RE. If the proposed mechanism is correct, it will be possible to infer the magnetospheric cavity shape from the measured periodicity of the tail oscillations during northward IMF. ¿ 2001 American Geophysical Union