NONBOUNCING PC-1 WAVE BURSTS

On April 11, 1986, at about 0600 UT a long Pc 1 wave event of the hydr omagnetic chorus type started on the ground, as registered by the Finn ish pulsation magnetometer network. The main pulsation band at about 0 .3 Hz was observed for several hours. Soon after start, this band smoo thly extended to higher frequencies, forming another separate wave ban d which finally reached up to 0.5 Hz. During the event the Viking sate llite was on its southbound pass over Scandinavia, close to the MLT se ctor of the ground network. From 0650 until 0657 UT, Viking observed a chain of Pc 1 bursts with increasing frequency. The strongest bursts could be grouped into two separate wave regions whose properties diffe red slightly. The higher-latitude region had a frequency of 0.3 Hz, we ll in agreement with the main Pc 1 band on the ground. The lower-latit ude region contained the highest frequencies observed on the ground at about 0.5 Hz. The latitudinal extent of both wave regions was about 0 .5 degrees. They had slightly different normalized frequencies, Alfven velocities, and repetition periods. Most interestingly, the repetitio n periods of both wave sources were too short for the bursts to be due to a wave packet bouncing between the two hemispheres. The results gi ve new information about the high-latitude Pc 1 waves, showing that th ey can consist of separate repetitive but nonbouncing bursts. We sugge st that the long-held bouncing wave packet hypothesis is generally inc orrect and discuss two alternative models where the burst structure is formed at the equatorial source region of the waves.