[P-EM05_2PO1] Characteristics of atmospheric waves in the mesosphere-lower thermosphere (MLT)

[PEM05-P05] Lamb波と熱圏にトラップされた重力波のカップリング

*西田 究¹、小林 直樹²、深尾 良夫³ (1.東大地震研、2.ISAS, JAXA、3.IFREE, Jamstec)

キーワード：Atmospheric Lamb wave, Thermospheric gravity wave

Lamb waves of the Earth's atmosphere in the millihertz band have been considered as transient phenomena excited only by large events. Nishida et al. (2014) showed the first evidence of background Lamb waves in the Earth's atmosphere from 0.2 to 10 mHz, based on the array analysis of microbarometer data from the USArray in 2012. The observations suggest that the probable excitation source is atmospheric turbulence in the troposphere. Theoretically, their energy in the troposphere tunnels into the thermosphere at a resonant frequency via thermospheric gravity wave because the Lamb-wave branch intersects that of thermospheric gravity waves at 3.5 mHz and that of acoustic waves trapped near the mesopause at 6.5 mHz [Garrett 1969]. The observed FK spectrum shows a local minimum of Lamb-wave amplitudes at around 3.5 mHz, where the Lamb-wave branch is crossed by the thermospheric gravity-wave branch. Coupled Lamb waves leak a certain amount of energy from the troposphere to the thermosphere, reducing the Lamb-wave amplitudes at the crossover frequency relative to those at neighboring frequencies, when their excitation sources exist in the troposphere. The energy tunnels from the troposphere to the thermosphere at the resonant frequency, although Lamb waves themselves cannot induce an upward flux [Lindzen 1972]. The RMS amplitudes of the coupled modes are estimated to be 0.3 m/s at 150 km and 0.1 m/s at 120 km, respectively. These modes might contribute to the thermosphere energy balance by heating via viscous dissipation [Hickey et al. 2001]. The amplitude suggests that the Lamb waves partly contribute to the excitation of thermospheric wave activity associated with severe convection activity [Hunsucker1982].