STARE radar observations of a Pg pulsation are used to study the spatial variation of the hydromagnetic wave electric fields in the ionosphere. Both the north-south and east-west electric fields appear to exhibit resonant behavior. The oscillation is confined to approximately 2¿ of latitude and both components have similar amplitude maxima of order 40 mV m-1. The longitudinal phase variations yield an azimuthal wave number m~17 and the corresponding ionospheric phase velocity is 8 km s-1 westward. Although apparently resonant, the magnitude of the east-west electric field is considerably larger than for previously reported resonant Pc 5's thought to originate in the Kelvin-Helmholtz instability on the magnetopause. It is suggested that the Pg may be an even mode oscillation generated by a bounce resonance instability.