SPATIAL AND TEMPORAL CHARACTERISTICS OF GIANT UNDULATIONS

A giant undulation event observed at SANAE, Antarctica (L = 4) is repo rted. During the event the geomagnetic activity was high, as was the c ase in previously reported observations of giant undulations. Data wer e recorded by all-sky and small field-of-view TV cameras, from 2346 UT on September 10, to 0007 UT on September 11 (MLT similar to 2200 h) 1 987. The fine temporal and spatial resolution of the TW data made a de tailed study of the undulation possible. The average wavelength was 17 0 km and the amplitude of the wave increased from 70 km to 140 km in 1 0 minutes. Wave propagation, measured at different positions on the un dulation, was duskward with a phase speed of 540 to 650 m/s. An inner structure, propagating at a speed of 2.5 to 4.0 km/s, about 3 times as fast as the undulation form, was observed just poleward of the undula tion boundary, indicating the presence of strong velocity shear. The l inear theory of the velocity shear instability is examined as a possib le generation mechanism on the basis of these observations.