Pulsating electric fields in the Pc 5 frequency range have been recorded at magnetically conjugate locations in the magnetosphere and in the ionosphere. This coordinated investigations has been conducted with the geostationary satellite GEOS 2 in the vicinity of the geomagnetic equator and STARE backscatter radar facility located in northern Scandinavia Such simultaneous electric field measurements can be understood in the light of the field line resonance theory. The profile of the resonant region is described, and the polarization of the pulsating field is defined. It is shown that the phase difference between the magnetospheric and ionospheric signals and the determination of the spacecraft invariant magnetic latitude are consistent with theoretical predictions made for the fundamental mode.