Solar cycle variation of the dominant frequencies of Pc3 geomagnetic pulsations at L = 1.6

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Abstract
The variation of the spectral characteristics of daytime Pc3 geomagnetic pulsations recorded at L'Aquila (Italy, L ∼ 1.6) during one solar cycle (1985–1994) is investigated on a yearly time scale. We find in the frequency domain two peaks in the event distribution for the H component during years of minimum solar activity and a single peak during years of maximum solar activity. These features can be interpreted in terms of the solar cycle variation of the predicted dominant frequency of the main driving source (upstream waves) as well as of the local fundamental field line eigenfrequency. The dominant frequency for the D component also changes according to the expected solar cycle variation of the dominant source frequency. The solar cycle variation of the estimated fundamental field line eigenfrequency is consistent with a general increase of the plasmasphere density along the local field line by a factor of ∼2 from minimum to maximum solar activity which is in agreement with theoretical models.

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