We report here a unique Pc 5 band quasi-electrostatic wave event (~300-s period) observed near local geomagnetic midnight at an invariant latitude of 75¿. The electric field signal was obtained from one of the eight high-latitude balloon payloads launched above the south geographic pole during the south Pole Balloon Campaign in the 1985--86 austral summer. The balloon payloads were instrumented with double-probe electric field detectors and bremsstrahlung X ray detectors. The electric field data from one flight of particular interest have been compared with ground-based magnetometer and micropulsation data in an attempt to understand the nature of the wave event. The Pc 5 waves were linearly polarized in the electric field, the electric field components had amplitudes of 20 to 30 mV/m, and the event persisted for an interval of more than 3 hours from 0000 to 0330 UT (2030 to 2400 MLT) on December 22, 1985. The magnetic activity was quiet during this time period. Detailed power spectra are presented in the paper. No evidence was found suggesting that the event was produced by an artifact. The event was not associated with atmospheric natural waves, weather processes, or upward propagating gravity waves. The event was produced in the ionosphere by a process other than the convection of irregularities. We suggest that ULF magnetosonic waves originating at the magnetopause produced the signals that were observed.