ULYSSES OBSERVATIONS OF A DISCRETE WAVEPACKET UPSTREAM OF AN INTERPLANETARY SHOCK

We present a single example of a previously unidentified type of upstr eam wave structure, found several minutes ahead of a forward propagati ng, oblique interplanetary shack observed on day 6, 1992, using Ulysse s magnetometer data. The wave appears as an isolated high frequency wa vepacket associated with a larger scale compressive structure. In the region between the wavepacket and the shock the magnetic field is mode rately, but significantly, compressed and deflected suggesting the exi stence of a foreshock. The wavepacket is dispersive and wavelet analys is shows a near linear decrease in frequency from 0.3 to 0.1 Hz, conti nuing across the large scale structure which is assumed to be the sour ce of these waves. We suggest that the feature is similar in nature to the shocklet and associated discrete wavepacket commonly found in pla netary foreshocks, but further examples are required before a definite conclusion can be reached.