On November 30, 1982, two large-amplitude Pc 5 pulsations were observed at Tromso, Norway, by both the European Incoherent Scatter (EISCAT) radar and ground-based magnetometers. The pulsations were excited by a sudden impulse. Their amplitude subsequently decreased, allowing a damping rate to be determined. Estimates of the height-integrated Pedersen conductivity obtained from the EISCAT data permit theoretical pulsation damping rates to be predicted. However, the theoretical damping rates are much greater than those actually measured, which indicates that energy was continuously fed into the field line resonance after its onset. The most likely source of this energy is coupling from a hydromagnetic cavity mode. If this is the case, the measured pulsation damping rate is controlled by the damping of the cavity mode rather than energy dissipation from the field lines resonance. An upper limit of &ggr;/&ohgr;=0.08 is obtained from the coupling efficiency between the two modes. The energy dissipated in the ionosphere by these pulsations was ~4¿1012 J. To store this energy in the cavity mode requires an initial c avity mode amplitude of only ~0.4 nT. Long-period magnetic perturbations of this amplitude would be difficult to detect in existing spaceborne magnetometer data sets. ¿ American Geophysical Union 1987