Eighteen months of data provided by the magnetometer and energetic particle spectrometer on board the geosynchronous satellite GEOS 2 were used to investigate compressional ULF waves with frequencies of a few megahertz associated with large anticorrelated modulations of the ion pressure. The events are classified into two groups according to the behavior of the background magnetic field/plasma configuration at the onset of the oscillations. For one group the oscillation onset is preceded by a sharp increase of the plasma energy density due to the arrival of drifting ion bunches at the satellite and a corresponding diamagnetic decrease of the magnetic field strength. The other group is observed in a steady configuration in the presence of an enhanced ring current. Besides striking differences between the two groups as regards the average event duration and wave frequency, as well as the phase relation between electron and ion flux modulations, we present evidence that both groups of events are of anti-symmetric structure with respect to the geomagnetic equator (where symmetry is defined according to the field line displacement) and have a small field-aligned wavelength. Furthermore, for both groups the frequency variation with local time and the azimuthal propagation are closely connected with the ion magnetic gradient curvature drifts, and the waves are associated with a plasma distributed anisotropically in velocity space. Therefore the waves most likely represent the excitation of a single wave mode generated via wave-particle interactions. Using plasma parameters measured in situ we calculated the growth rate of different hydromagneic wave modes. It was found that the plasma is able to support a wave mode similar to the drift mirror mode with a large parallel wave number, if the effects of a strongly curved magnetic field are taken into account. ¿ American Geophysical Union 1990