A pulsation event with a period of 6--8 min has been observed simultaneously by the geostationary satellite GEOS 2 and two balloon located in the area magnetically conjugate to the satellite. The balloons were equipped with electric field and X ray instruments. The pulsation event occurred in the magnetic local time sector 0600--1100 determined by ground-based meaurements. The satellite instruments observed the signature of a magnetosonic wave having a radially inward directed Poynting vector. The electric field associated with the wave was closely correlated with the electric field detected by the balloon instrument near the footprint of the magneic field line through GEOS 2. The ground-based observations show that the wave phase pattern was north-south straited and that it moved from east to west. The balloon x-ray measurements showed a strongly pulsating particle precipitation, in close correlation with the intensity of the energetic electron flux and the VLF electromagnetic radiation measured on the satellite, suggesting a wave-particle interaction process. The pulsations seem to originate beyond the geosynchronous orbit. The study demonstrates the usefulness of simultaneous measurements with a geostationary satellite and instruments in the ground area magnetically conjugate to the satellite.