Auroral zone observations often show significant ULF power. We have analyzed auroral and plasma sheet observations during two prolonged periods of strongly southward and relatively steady interplanetary magnetic field (IMF). We find evidence that auroral poleward boundary intensifications (PBIs), which have large intensity and occur repetitively throughout such periods, may be a manifestation of a large-scale ULF oscillation mode that strongly perturbs the plasma sheet and the auroral ionosphere. If this is correct, then ULF modes would be a major component of tail dynamics, of magnetosphere coupling to the ionosphere, and of auroral zone disturbances during periods of enhanced convection. They would simultaneously affect a large region of the nightside, extending along auroral zone field lines from the ionosphere to the equatorial plasma sheet and extending from field lines that lie near the magnetic separatrix to, at times, as close to the Earth as synchronous orbit. They would also occasionally have amplitudes as large as the changes that occur in association with other auroral zone disturbances such as substorms. Here we have found peak-to-peak amplitudes as high as several hundred nanoteslas in ground X, an order of magnitude in synchronous energetic proton fluxes, ~20--40 nT in synchronous magnetic field components, ~20 nT in tail magnetic field components, ~1000 km/s in tail flow speeds, and ~400 m/s in ionospheric flow speed. We find evidence for significant power at 0.5--0.7 mHz (~25--30 min period), significant power at a possible second harmonic (~1.1--1.3 mHz), and power at frequencies that could be higher harmonics simultaneously within the auroral ionosphere and within the nightside plasma sheet.