A detailed analysis of HF radar data of a ULF pulsation event in the postmidnight sector on January 11, 1989, has been carried out using techniques which allow the instantaneous amplitude and phase to be determined as functions of geomagnetic latitude, longitude, and time. Field line resonances with several different frequencies occur simultaneously at different latitudes. These can be associated with cavity mode frequencies of 1.3 mHz, 1.9 mHz, 2.7 mHz, and 3.3 mHz. In addition there is a resonance at 0.8 mHz which does not fit well with a cavity picture. These frequencies are constant to better than 10% over a local time period of nearly 4 hours. They show a packet structure as would be expected if they were triggered by a succession of impulses. The phase changes arbitrarily from packet to packet, but the frequency remains constant. The position of the maximum of the resonance as a function of time changes systematically. It is shown that this arises as the length of the field line changes with time; the resonance remains on the field line having appropriate length and Alfv¿n speed. The field-aligned currents driven by the resonances can be as large as 5 &mgr;A m-2 at ionospheric heights. The data support a picture of modes driven by solar wind impulses. It may be more appropriate to speak of a waveguide rather than a cavity with the phase velocity of the mode matching the velocity of the impulse along the magnetopause. A difficulty associated with this picture is that the great reproducibility of the frequencies is not consistent with the variability of the magnetopause, which forms one of the boundaries of the assumed resonator. It is, however, difficult to conceive of other resonators, for example in the magnetotail, which would provide a better explanation of the observations. ¿ American Geophysical Union 1992