Correlated ground magnetic and geostationary satellite electric field measurements of four ULF pulsation events occurring at different local times within the Earth's magnetosphere are analyzed with special emphasis on the influence of the ionosphere on ground-based observations. Using a simplified rule for the projection of the electric field observations down to the ionosphere, we find two occasions where the ionospheric rotation angle is about 90¿ and two occasions where it significantly deviates from this value. A possible explanation of these results is given in terms of a recently developed theory concerning the influence of an ionosphere with arbitrary nonuniform horizontal conductivity distribution and in terms of propagation of hydromagnetic waves in noncoplanar magnetic field geometries. The need for further study of this latter point is strongly emphasized. ¿ American Geophysical Union 1987