The Stare (Scandinavian twin auroral radar experiment) auroral radar system has been used to measure ionospheric electric fields associated with Pc 5 geomagnetic pulsations. With this system, electric fields are derived from the drift velocity of radar auroral irregularities. The spatial resolution is 20 km over a 200,000-km2 grid, and the temporal resolution is 20 s. It has been found that the oscillating electric field associated with a hydromagnetic field line resonance produces poleward moving, bandlike regions of radar aurora, which are aligned in the east-west direction. The drift of the irregularities within these bands is alternately eastward and westward. The Stare electric field data have been used in conjunction with the Biot-Savart Law and an assumed height-integrated conductivity of 8--10 &OHgr;-1 to calculate the ground magnetic disturbance. It has been found that the H and Z are well predicted, whereas D is generally underestimated. By Fourier analyses of the Stare data it is found that the half-power latitudinal width of the field line resonance is typically 100 km in the ionosphere. Moreover, the north-south electric field undergoes a 180¿ phase shift about the resonance as predicted by theory. The data have been used to estimate equatorial plasma densities for 6<L<, and values of the order of 20 cm-3 have been obtained. However, these determinations are strongly affected by distortions of the geomagnetic field from a dipolar geometry. In summary, (1) the experimental results strongly support the hydromagnetic field line resonance theory of pulsations: (2) the magnetic polarization ellipse is indeed rotated through 90¿ by the ionosphere: (3) the phenomena previously observed by auroral radar workers in association with Pc 5 pulsations were related to the electric field of the hydromagnetic wave near resonance: and (4) auroral radar measurements can be used to estimate the equatorial magnetospheric plasma density in the region 5<L<8.