Abstract
Recently free oscillations of the earth have been reported to be continuously excited in the absence of great earthquakes. Average amplitudes of those modes are about 0.5 nano gal in the mHz band and the amplitudes vary annually or semi-annually about 15%. Modes at frequencies of 3.7, 4.4, 5.1 and 6.1 mHz have remarkable excess in amplitudes. At those frequencies, the seismic Rayleigh branch is crossing over the infrasound branches of the atmosphere. To explain those excess, we calculated the Rayleigh modes and the acoustic modes in mHz band to examine the effect of atmospheric structure on the normal modes. We found that the excess amplitudes of the modes at the branch crossings can be reproduced only in the case that the excitation source is at the bottom of the atmosphere. These results show that the atmosphere plays an important role in the excitation of the observed background free oscillations.
