BALLOON OBSERVATIONS OF NIGHTSIDE PC 5 QUASI-ELECTROSTATIC WAVES ABOVE THE SOUTH-POLE

We report here a unique Pc 5 band quasi-electrostatic wave event (appr oximately 300-s period) observed near local geomagnetic midnight at an invariant latitude of 75-degrees. The electric field signal was obtai ned from one of the eight high-latitude balloon payloads launched abov e the south geographic pole during the South Pole Balloon Campaign in the 1985-86 austral summer. The balloon payloads were instrumented wit h double-probe electric field detectors and bremsstrahlung X ray detec tors. The electric field data from one flight of particular interest h ave been compared with ground-based magnetometer and micropulsation da ta in an attempt to understand the nature of the wave event. The Pc 5 waves were linearly polarized in the electric field, the electric fiel d 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 240 0 MLT) on December 22, 1985. The magnetic activity was quiet during th is time period. Detailed power spectra are presented in the paper. No evidence was found suggesting that the event was produced by an artifa ct. The event was not associated with atmospheric neutral waves, weath er processes, or upward propagating gravity waves. The event was produ ced in the ionosphere by a process other than the convection of irregu larities. We suggest that ULF magnetosonic waves originating at the ma gnetopause produced the signals that were observed.