We analyze a Pc5 ULF pulsation event that occurred on 12 May, 1999, following an extended interval of extremely low solar wind dynamic pressure (ram pressure dropped to ~0.2nPa; bow shock standoff position estimated at ~60RE). Using the IMAGE magnetometer array, we show that the extended magnetosphere at this time gave rise to an Alfv¿n continuum of field line oscillations in the outer magnetosphere of anomalously low frequency, consistent with field lines mapping out to large distances within the enlarged magnetospheric cavity. We demonstrate that the pulsation event was characterized by discrete, low frequency signals consistent with cavity/waveguide modes, and that these discrete modes couple to high latitude field line resonances during the event. Our observations suggest that cavity/waveguide modes continue as the dominant source of energy input to geomagnetic pulsations even during intervals when the magnetosphere is abnormally extended. ¿ 2000 American Geophysical Union