HYDROMAGNETIC BACKGROUND OF THE MAGNETOSPHERE AND GYRORESONANCE SWINGING OF A GIANT PC-2 WAVE EVENT

The spectral and temporal characteristics of a ''giant'' Pc 2 (tau app roximately 7 s) hydromagnetic wave event of duration approximately 110 s have been reanalyzed, as have the electromagnetic oscillations gene rated in the magnetosphere in an interval of about 45 min preceding an d following the event. The event, which was recorded near local midnig ht at two mid-latitude locations, is, to the best of our knowledge, th e largest such wave event reported to date. Previous analyses and inte rpretation of the event have centered on the bounce resonance generati on of this packet of oscillations (Lanzerotti et al., 1986). The reexa mination of the event, as well as other oscillations of the magnetopla sma, has caused us to identify six distinguishable spectral frequency bands which appear to encompass die Pc 2 wave packet and its evolution in time as well as to look at the development of the process as a who le. We present the details of these temporal and spectral studies and provide evidence that the Pc 2 event is probably produced by the gyror esonance swinging: amplification of the oscillatory magnetoplasma back ground in the frequency band approximately 0.1-0.2 Hz. The analysis su ggests that the oscillatory background could be produced by impulse-li ke disturbances. The possible source(s) of such disturbances and their nature require further study.