Spectral structure of mid-latitude Pc3-4 geomagnetic pulsations.

Methodical examination of dynamic spectra of Pc3-4 geomagnetic pulsation activity recorded over 25 months at an L=2.1 ground station shows that activity can be sorted into distinct phenomenological classes based on the spectral structure of the signal. The most common features observed were sustained, relatively narrowband signals exhibiting periodic variation in amplitude, comprising oscillations at two or more nearby frequencies. Short duration wavepackets of sinusoidal oscillations were also found to be very common structures. Often, intervals of irregular, quite broadband oscillations in the Pc3-4 range were observed, with durations of a few minutes to several hours. Frequency structured activity comprising multiple spectral components, or pulsations whose frequency changed suddenly or smoothly, were observed on average every second day. Finally, intervals of sustained, sinusoidal monochromatic oscillations were observed on infrequent occasions. It is unlikely that all these diverse spectral features result solely from the dynamic response of the magnetosphere to varying conditions of geomagnetic agitation. It would seem that the respective excitation and generation mechanisms play at least some part in determining the spectral structure of pulsation signals recorded on the ground. Consequently, careful interpretation of dynamic spectra can assist in understanding both the mechanisms by which the pulsations are generated, and the temporal evolution of these generation processes with the level of geomagnetic agitation. It is concluded that there are good reasons to make a systematic study of dynamic spectra an integral part of any comprehensive analysis of geomagnetic pulsation phenomena.