Planetary Oscillations Excited by Atmospheric Turbulence

在那些日子里，地球的自由振荡被认为是由地震以外的因素不断激发的。可能的机制是1996年由Koabayashi提出的，即由大气湍流激发。为了确认其作用机理，我们进行了理论和分析研究。如果得到证实，这项研究将对行星探测产生重大影响，因为对火星和金星等其他地震不活跃行星的地震调查将成为现实。因此，进一步了解地球连续自由振荡的本质是很重要的。我们收集了50个安静（噪声较小）台站的连续地震数据，并计算了地震安静日它们之间的交叉谱。然后，从谱中得到了波数-频率平面上的谱。它清晰地显示了基本球模的分支和微弱的第一泛音分支。不仅对频率而且对波数进行光谱解析的优点是可以更准确地估计背景震动中自由振荡的贡献。结果表明，连续自由振荡对背景光谱的贡献最大。此外，为了给大气激励机制提供更多的证据，我们评估了与大气模态耦合的球体模态0S29的振幅。大气模式本身可以被大气中的湍流激发。我们发现，0S29的观测振幅比其他模态的振幅高出约40%，这可以通过与湍流激发的连续激发大气模态的耦合来解释。因此，耦合模不仅可以由大气直接激发，也可以通过大气模间接激发。结果表明，我们通过本研究进一步证实了大气激发机制。

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.