Ermolenko S. I., Shved G. M.
Spheroidal Earth’s free oscillations 0Sn, where n is the number of oscillation nodes along a meridian between the poles, are observed during seismically quiet days. The seismometer measurements of these background Earth’s free oscillations with n = 12—65, located at frequencies of 2—7 mHz (periods ~8—2 min), show that the Earth’s free oscillations are caused by atmospheric processes. The following mechanism for excitation of the Earth’s free oscillations is proposed. During strong storms high waves generate oceanic internal gravity waves, which excite, in turn, Earth’s free oscillations acting on the seabed. The present study is devoted to the search for the excitation source of background Earth’s free oscillations 0Sn, which is located at the frequency of about 0.3 mHz (period of about 54 min) and is the lowest frequency Earth’s free oscillations. We have used the continuous measurements of the vertical (Z) output signal of STS-2 seismometer at Collm, Germany (51.3°N, 13.0°E), for the full year of 2002. The spectral analysis was applied over the 5-day window sliding along the annual series of the seismometer measurements with 1-day step. The values of power spectral density at the frequencies of the 0Sn multiplet and the frequencies close to them are matched with the values of the Arctic oscillation index, which characterizes the intensity of dynamic perturbation of the atmosphere in the Northern hemisphere. It is found that the correlation coefficients between these values are positive and statistically valid. So, it is shown for the first time that the background Earth’s free oscillations 0Sn can be excited by dynamic processes in the atmosphere, just as it is for highfrequency Earth’s free oscillations.