Resonant oscillations between the solid earth and the atmosphere

Continuously excited free oscillations of the whole Earth have been found in the ground noise in a frequency band of long-period surface waves. Here we report evidence of an annual variation of this phenomenon, indicating that the excitation source is not within the solid Earth. There is evidence that these seismic free oscillations resonate with acoustic free oscillations of the atmosphere. The observed amplitudes suggest that the excitation source is at or just above Earth's surface.