Planetary Oscillations Excited by Atmospheric Turbulence

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In those days, Earth's free oscillations are known to be continuously excited by other than earthquakes. The probable mechanism is one proposed by Koabayashi in 1996, that is excitation by atmospheric turbulence. To confirm the mechanism, we have done theoretical and analytical studies. If it is proven, this study will be a big impact to planetary exploration because seismological investigation of other seismically inactive planets such as Mars and Venus becomes realistic. So it is important to understand further the nature of the continuous Earth's free oscillations.We gathered continuous seismic data from 50 quiet (less noisy) stations, and calculated cross-spectra between them on seismically quiet days. Then, from the spectra, we finally obtained a spectra on wave number - frequency plain. It clearly shows the branch of the fundamental spheroidal modes and faint first overtone branch. The merit of resolving spectra with respect to not only frequency but also to wave number is giving more accurate estimate of contribution of free oscillations in background tremors. As a result, we firstly clarify main contribution to the background spectra in the mHz band is the continuous free oscillations.In addition, to give more evidence of the atmosphericexcitation mechanism, we evaluated amplitude of the spheroidal mode 0S29 coupled with atmospheric modes. Atmospheric modes themselves can be excited by the turbulence in the atmosphere. We showed that the observed amplitude of 0S29 which show about 40% excess to those of other modes can be explained by coupling with continuously excited atmospheric modes excited by the turbulence. Thus the coupled modes are excited not only by the atmosphere directly but also excited via atmospheric modes indirectly. In the result, we have further confirmed the atmospheric excitation mechanism through this study.