A WHISTLER STUDY OF THE BULGE REGION OF THE PLASMAPAUSE

Knee whistlers and plasmaspheric whistlers recorded by the VLF goniometer receiver at Halley, Antarctica on 24th June 1977 have been analysed. The knee whistlers were received between 1900 UT and 2300 UT (1630 MLT and 2030 MLT) providing good coverage of the duskside region of the plasmapause. Estimates of the plasmapause position range from L=4.4 at 1700 UT to L=4.7 at 1930 UT and then back to L=4.2 at 2300 UT, as the observing station moved into and through the bulge. Arrival azimuths, nose frequencies and nose travel times were scaled for about 30 ducts inside the plasmasphere and about 10 ducts outside. The azimuths give longitudinal information which, when combined with the determination of the equatorial L-value of a duct using the conventional whistler method, enables its position to be located. Since our analysis technique enables us to process rapidly the large number of whistlers which were observed during this period, we have been able to produce detailed maps of structure near to the plasmapause. Accompanying the anomalously decreasing plasmapause radius after 1730 MLT, where the evening bulge would normally occur, and probably related to it were observed rapid inward drifts of the whistler paths