Simultaneous quasi-periodic variations of electrons (Ee>22 keV), ions (Ei>27 keV), and the geomagnetic field in the Pc 5 period range were observed on board the geostationary satellite geos 2. Fifty-four events occurred between August 1978 and July 1979. Two types of events could be distinguished. During the first type, electrons and ions reached their flux maxima and minima simultaneously ('in-phase events'). During events of the second type the electron flux had minima at the ion flux maxima and vice versa ('out-of-phase events'). During all events the total magnetic induction had minima at the time of the ion flux maxima and vice versa. The in-phase events occurred preferetially around noon. They had longer average periods and durations than the out-of-phase events that appeared around dusk. The phase relationship between electrons and ions was found to depend on the slope of the electron pitch angle distributions. For pitch angles &agr;><0. The ion intensity variations were associated with azimuthal asymmetries that can be interpreted as the east to west movement of regions of intense ion fluxes. The observations are in reasonable agreement with the predictions of the drift mirror instability theory. In terms of this theory the two different types of quasi-periodic events can be regarded as the result of different responses of the electrons to the disturbance conditions around noon and dusk. The drift mirror instability is associated with drifting energetic proton bunches that generate Alfv¿n waves. An estimate of the wave amplitudes yielded values similar to those actually observed.