Observations of an anomalously low frequency Alfven continuum in an abnormally expanded magnetosphere

We analyze a Pc5 ULF pulsation event that occurred on 12 May, 1999, followi ng an extended interval of extremely low solar wind dynamic pressure (ram p ressure dropped to similar to0.2nPa; bow shock standoff position estimated at similar to 60R(E)). Using the IMAGE magnetometer array, we show that the extended magnetosphere at this time gave rise to an Alfven continuum of fi eld line oscillations in the outer magnetosphere of anomalously low frequen cy, consistent with field lines mapping out to large distances within the e nlarged magnetospheric cavity. We demonstrate that the pulsation event was characterized by discrete, low frequency signals consistent with cavity/wav eguide modes, and that these discrete modes couple to high latitude field l ine resonances during the event. Our observations suggest that cavity/waveg uide modes continue as the dominant source of energy input to geomagnetic p ulsations even during intervals when the magnetosphere is abnormally extend ed.