A frequently observed feature in the electric field data on board GEOS 2 is the sudden occurrence of large-amplitude Pc 4--5 waves lasting from a few tens of minutes up to a few hours. For the events we have chosen it was found that the large-amplitude events (≥2 mV/m) are often accompanied by magnetic field data containing the signature of a compression of the magnetosphere. The occurrence of the compression coincides with the recording of an ssc (sudden storm commencement) on ground. The electric field measurements suggest that the E¿B/B2 drift rapidly and temporarily rotates into a tailward plasma drift at 6.6 RE. The transient disturbances of the magnetopause also changed the widely scattered frequency distribution of the nearly always present fluctuations in the Pc 4--5 range into a narrow-band distribution, with frequently large amplitudes, after the occurrence of the ssc. The frequency distribution reveals a weak dependence on the local time such that the frequencies are close to 4 mHz near noon and about 3 mHz near dawn and dusk. The rotational sense of polarization is left-handed between 0200 and 1200 LT and right-handed between 1400 and 2200 LT: a transition region exists between 1200 and 1400 LT. We argue that the transverse waves are the fundamental modes of the field line resonances while the compressional modes might be the fundamental modes and the second harmonics of the cavity resonances or the ''plasmasphere'' modes. ¿ American Geophysical Union 1989