The power spectra of geomagnetic field fluctuations in the hydromagnetic wave band of periods 20–600 s have been systematically evaluated from data acquired at L'Aquila, Italy (geomagnetic invariant longitude Φ = 94.4°; invariant latitude Λ ∼ 42.5°; L ∼ 1.6) and Green Hill, R.I. (Φ = 5°; Λ ∼ 52°; L ∼ 2.8) during July 1985. The average levels of the ULFpower in different frequency bands have been compared. Autocorrelations of power levels for the data at the two stations show a clear 24 h recurrence period at L'Aquila in the bands 20–30 s and 30–45 s, while the same periodicity is clearly observed at Green Hill for the 30–45 s and 45–150 s bands. The results of the cross correlation analysis of the data from the two stations indicate that there is a clear U.T. dependence for geomagnetic power for periods between 45 and 600 s, suggesting global source influences on the power levels. The power in the period band 30–45 s appears to be better correlated on a L.T. basis suggesting a similar diurnal pattern at the two stations of the 24 h modulation in this range of frequency.
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