日本地球惑星科学連合2018年大会

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[EE] ポスター発表

セッション記号 S (固体地球科学) » S-IT 地球内部科学・地球惑星テクトニクス

[S-IT22] 核-マントルの相互作用と共進化

2018年5月22日(火) 10:45 〜 12:15 ポスター会場 (幕張メッセ国際展示場 7ホール)

コンビーナ:飯塚 毅(東京大学)、渋谷 秀敏(熊本大学大学院先端科学研究部基礎科学部門地球環境科学分野)、土屋 卓久(愛媛大学地球深部ダイナミクス研究センター、共同)、太田 健二(東京工業大学大学院理工学研究科地球惑星科学専攻)

[SIT22-P35] The effect of an azimuthal background magnetic field on waves in a stably stratified layer at the top of the Earth's outer core

*中島 涼輔1吉田 茂生2 (1.九州大学 大学院理学府 地球惑星科学専攻、2.九州大学 大学院理学研究院 地球惑星科学部門)

キーワード:地球外核最上部安定成層、MHD浅水波

We investigated waves in a stably stratified thin layer in a rotating sphere with an imposed magnetic field. This represents the stably stratified outermost Earth's core or the tachocline of the Sun. Recently, many geophysicists focus on the stratification of the outermost outer core evidenced through seismological studies (e.g. Helffrich and Kaneshima, 2010) and an interpretation of the 60-year geomagnetic secular variations with Magnetic-Archimedes-Coriolis (MAC) waves (Buffett, 2014).
Márquez-Artavia et al.(2017) studied the effect of a toroidal magnetic field on shallow water waves over a rotating sphere as the model of this stratified layer. On the other hand, MAC waves are strongly affected by a radial field (e.g. Knezek and Buffett, 2018). We added a non-zero radial magnetic perturbation and magnetic diffusion to Márquez-Artavia et al.(2017)'s equations. Unlike their paper's formulation, we applied velocity potential and stream function for both fluid motion and magnetic perturbation, which is similar to the first method of Longuet-Higgins(1968).
In the non-diffusive case, the dispersion relation obtained with the azimuthal equatorially symmetric field (Bφ(θ) ∝ sinθ, where θ is colatitude) is almost the same as Márquez-Artavia et al.(2017)'s result, which includes magneto-inertia gravity (MIG) waves, fast magnetic Rossby waves, slow MC Rossby waves and an unexpected instability. In particular, we replicate the transition of the propagation direcition of zonal wavenumber m=1 slow MC Rossby waves from eastward to westward with increasing Lamb parameter (ε=4Ω2a2/gh, where Ω, a, g and h is the rotation rate, the sphere radius, the acceleration of gravity and a equivalent depth, respectively) and Lehnert number (α=vA/2Ωa, where vA is Alfvén wave speed). As a consequence, fast magnetic Rossby waves and slow MC Rossby waves interact, and the non-diffusive instability occur.
Next, we are examining the case with an equatorially antisymmetric background field, which is more realistic in the Earth's core. In this case, if the magnetic diffusion is ignored, the continuous spectrums appear owing to Alfvén waves resonance (similar to the continuous spectrums in inviscid shear flow, e.g. Balmforth and Morrison, 1995). To solve this difficulty, our numerical model includes the magnetic diffusion term.