DISPERSIVE PC1 BURSTS OBSERVED BY FREJA

Electric field measurements by the Freja double probe sensor are used to study equatorially generated ion cyclotron waves, also called Pc1 p ulsations. We have examined the global occurrence and spectral propert ies of these waves in the upper ionosphere during 12-hour period on No v. 18, 1992, when a long chain of structured Pc1 waves (pearls) was ob served on ground. In agreement with ground observations, Pc1 waves wer e found to occur as short bursts of 10-25 s in early morning to postno on MLT sector. Most Pc1 activity was detected within a small latitudin al range, extending from 60-degrees CGMlat at dawn to 630 CGMlat at no on. The latitudinal width of the source was only about 0.5-degrees CGM lat. Observations give evidence for a plasmapause connected source reg ion that was several hours wide in MLT and active during many hours. O ne burst displayed a fully developed classical dispersive Pc1 pearl, n ow detected for the first time above the ionosphere. In all studied Pc 1 events, two spectral maxima (bands) were observed. The longer Pc1 wa ve bursts showed evidence for a small time delay between the lower and upper frequency bands, unveiling a new dispersive phenomenon.