An analysis of micropulsation events at a low-latitude station during 1985

Geomagnetic field fluctuations detected at L'Aquila (L ∼ 1.6) during 1985 have been investigated on a statistical basis in the range of frequencies 7.8–234 mHz (Pc2, Pc3 and Pc4). The most significant events have been selected by means of a comparison between dynamic power spectra and the background spectrum. Dayside events, which perturb mainly the North-South component, tend to occur in two separate ranges, namely P1 = 25 ± 5 s, and P2 = 13 ± 2 s. The morphological differences between these groups of oscillations appear consistent with a wide source spectrum peaked at P1 (possibly upstream waves generated in the solar wind foreshock region) and a resonant mechanism at P2 which might correspond to the fundamental eigenperiod of the local field-line. Night-time events show all the characteristics of Pi2 pulsations and no evidence for field-line resonance phenomena is supported by the experimental observations.